fix(streaming): filter <think> blocks from gateway stream consumer

Models like MiniMax emit inline <think>...</think> reasoning blocks in
their content field. The CLI already suppresses these via a state machine
in _stream_delta, but the gateway's GatewayStreamConsumer had no
equivalent filtering — raw think blocks were streamed directly to
Discord/Telegram/Slack.

The fix adds a _filter_and_accumulate() method that mirrors the CLI's
approach: a state machine tracks whether we're inside a reasoning block
and silently discards the content. Includes the same block-boundary
check (tag must appear at line start or after whitespace-only prefix)
to avoid false positives when models mention <think> in prose.

Handles all tag variants: <think>, <thinking>, <THINKING>, <thought>,
<reasoning>, <REASONING_SCRATCHPAD>.

Also handles edge cases:
- Tags split across streaming deltas (partial tag buffering)
- Unclosed blocks (content suppressed until stream ends)
- Multiple consecutive blocks
- _flush_think_buffer on stream end for held-back partial tags

Adds 22 unit tests + 1 integration test covering all scenarios.

tests/gateway/test_stream_consumer.py

@@ -680,3 +680,202 @@ class TestCancelledConsumerSetsFlags:
         # Without a successful send, final_response_sent should stay False
         # so the normal gateway send path can deliver the response.
         assert consumer.final_response_sent is False
+
+
+# ── Think-block filtering unit tests ─────────────────────────────────────
+
+
+def _make_consumer() -> GatewayStreamConsumer:
+    """Create a bare consumer for unit-testing the filter (no adapter needed)."""
+    adapter = MagicMock()
+    return GatewayStreamConsumer(adapter, "chat_test")
+
+
+class TestFilterAndAccumulate:
+    """Unit tests for _filter_and_accumulate think-block suppression."""
+
+    def test_plain_text_passes_through(self):
+        c = _make_consumer()
+        c._filter_and_accumulate("Hello world")
+        assert c._accumulated == "Hello world"
+
+    def test_complete_think_block_stripped(self):
+        c = _make_consumer()
+        c._filter_and_accumulate("<think>internal reasoning</think>Answer here")
+        assert c._accumulated == "Answer here"
+
+    def test_think_block_in_middle(self):
+        c = _make_consumer()
+        c._filter_and_accumulate("Prefix\n<think>reasoning</think>\nSuffix")
+        assert c._accumulated == "Prefix\n\nSuffix"
+
+    def test_think_block_split_across_deltas(self):
+        c = _make_consumer()
+        c._filter_and_accumulate("<think>start of")
+        c._filter_and_accumulate(" reasoning</think>visible text")
+        assert c._accumulated == "visible text"
+
+    def test_opening_tag_split_across_deltas(self):
+        c = _make_consumer()
+        c._filter_and_accumulate("<thi")
+        # Partial tag held back
+        assert c._accumulated == ""
+        c._filter_and_accumulate("nk>hidden</think>shown")
+        assert c._accumulated == "shown"
+
+    def test_closing_tag_split_across_deltas(self):
+        c = _make_consumer()
+        c._filter_and_accumulate("<think>hidden</thi")
+        assert c._accumulated == ""
+        c._filter_and_accumulate("nk>shown")
+        assert c._accumulated == "shown"
+
+    def test_multiple_think_blocks(self):
+        c = _make_consumer()
+        # Consecutive blocks with no text between them — both stripped
+        c._filter_and_accumulate(
+            "<think>block1</think><think>block2</think>visible"
+        )
+        assert c._accumulated == "visible"
+
+    def test_multiple_think_blocks_with_text_between(self):
+        """Think tag after non-whitespace is NOT a boundary (prose safety)."""
+        c = _make_consumer()
+        c._filter_and_accumulate(
+            "<think>block1</think>A<think>block2</think>B"
+        )
+        # Second <think> follows 'A' (not a block boundary) — treated as prose
+        assert "A" in c._accumulated
+        assert "B" in c._accumulated
+
+    def test_thinking_tag_variant(self):
+        c = _make_consumer()
+        c._filter_and_accumulate("<thinking>deep thought</thinking>Result")
+        assert c._accumulated == "Result"
+
+    def test_thought_tag_variant(self):
+        c = _make_consumer()
+        c._filter_and_accumulate("<thought>Gemma style</thought>Output")
+        assert c._accumulated == "Output"
+
+    def test_reasoning_scratchpad_variant(self):
+        c = _make_consumer()
+        c._filter_and_accumulate(
+            "<REASONING_SCRATCHPAD>long plan</REASONING_SCRATCHPAD>Done"
+        )
+        assert c._accumulated == "Done"
+
+    def test_case_insensitive_THINKING(self):
+        c = _make_consumer()
+        c._filter_and_accumulate("<THINKING>caps</THINKING>answer")
+        assert c._accumulated == "answer"
+
+    def test_prose_mention_not_stripped(self):
+        """<think> mentioned mid-line in prose should NOT trigger filtering."""
+        c = _make_consumer()
+        c._filter_and_accumulate("The <think> tag is used for reasoning")
+        assert "<think>" in c._accumulated
+        assert "used for reasoning" in c._accumulated
+
+    def test_prose_mention_after_text(self):
+        """<think> after non-whitespace on same line is not a block boundary."""
+        c = _make_consumer()
+        c._filter_and_accumulate("Try using <think>some content</think> tags")
+        assert "<think>" in c._accumulated
+
+    def test_think_at_line_start_is_stripped(self):
+        """<think> at start of a new line IS a block boundary."""
+        c = _make_consumer()
+        c._filter_and_accumulate("Previous line\n<think>reasoning</think>Next")
+        assert "Previous line\nNext" == c._accumulated
+
+    def test_think_with_only_whitespace_before(self):
+        """<think> preceded by only whitespace on its line is a boundary."""
+        c = _make_consumer()
+        c._filter_and_accumulate("  <think>hidden</think>visible")
+        # Leading whitespace before the tag is emitted, then block is stripped
+        assert c._accumulated == "  visible"
+
+    def test_flush_think_buffer_on_non_tag(self):
+        """Partial tag that turns out not to be a tag is flushed."""
+        c = _make_consumer()
+        c._filter_and_accumulate("<thi")
+        assert c._accumulated == ""
+        # Flush explicitly (simulates stream end)
+        c._flush_think_buffer()
+        assert c._accumulated == "<thi"
+
+    def test_flush_think_buffer_when_inside_block(self):
+        """Flush while inside a think block does NOT emit buffered content."""
+        c = _make_consumer()
+        c._filter_and_accumulate("<think>still thinking")
+        c._flush_think_buffer()
+        assert c._accumulated == ""
+
+    def test_unclosed_think_block_suppresses(self):
+        """An unclosed <think> suppresses all subsequent content."""
+        c = _make_consumer()
+        c._filter_and_accumulate("Before\n<think>reasoning that never ends...")
+        assert c._accumulated == "Before\n"
+
+    def test_multiline_think_block(self):
+        c = _make_consumer()
+        c._filter_and_accumulate(
+            "<think>\nLine 1\nLine 2\nLine 3\n</think>Final answer"
+        )
+        assert c._accumulated == "Final answer"
+
+    def test_segment_reset_preserves_think_state(self):
+        """_reset_segment_state should NOT clear think-block filter state."""
+        c = _make_consumer()
+        c._filter_and_accumulate("<think>start")
+        c._reset_segment_state()
+        # Still inside think block — subsequent text should be suppressed
+        c._filter_and_accumulate("still hidden</think>visible")
+        assert c._accumulated == "visible"
+
+
+class TestFilterAndAccumulateIntegration:
+    """Integration: verify think blocks don't leak through the full run() path."""
+
+    @pytest.mark.asyncio
+    async def test_think_block_not_sent_to_platform(self):
+        """Think blocks should be filtered before platform edit."""
+        adapter = MagicMock()
+        adapter.send = AsyncMock(
+            return_value=SimpleNamespace(success=True, message_id="msg_1")
+        )
+        adapter.edit_message = AsyncMock(
+            return_value=SimpleNamespace(success=True)
+        )
+        adapter.MAX_MESSAGE_LENGTH = 4096
+
+        consumer = GatewayStreamConsumer(
+            adapter,
+            "chat_test",
+            StreamConsumerConfig(edit_interval=0.01, buffer_threshold=5),
+        )
+
+        # Simulate streaming: think block then visible text
+        consumer.on_delta("<think>deep reasoning here</think>")
+        consumer.on_delta("The answer is 42.")
+        consumer.finish()
+
+        task = asyncio.create_task(consumer.run())
+        await asyncio.sleep(0.15)
+
+        # The final text sent to the platform should NOT contain <think>
+        all_calls = list(adapter.send.call_args_list) + list(
+            adapter.edit_message.call_args_list
+        )
+        for call in all_calls:
+            args, kwargs = call
+            content = kwargs.get("content") or (args[0] if args else "")
+            assert "<think>" not in content, f"Think tag leaked: {content}"
+            assert "deep reasoning" not in content
+
+        try:
+            task.cancel()
+            await task
+        except asyncio.CancelledError:
+            pass