fix(weixin): streaming cursor, media uploads, markdown links, blank messages (#8665)

Four fixes for the Weixin/WeChat adapter, synthesized from the best
aspects of community PRs #8407, #8521, #8360, #7695, #8308, #8525,
#7531, #8144, #8251.

1. Streaming cursor (▉) stuck permanently — WeChat doesn't support
   message editing, so the cursor appended during streaming can never
   be removed.  Add SUPPORTS_MESSAGE_EDITING = False to WeixinAdapter
   and check it in gateway/run.py to use an empty cursor for non-edit
   platforms.  (Fixes #8307, #8326)

2. Media upload failures — two bugs in _send_file():
   a) upload_full_url path used PUT (404 on WeChat CDN); now uses POST.
   b) aes_key was base64(raw_bytes) but the iLink API expects
      base64(hex_string); images showed as grey boxes.  (Fixes #8352, #7529)
   Also: unified both upload paths into _upload_ciphertext(), preferring
   upload_full_url.  Added send_video/send_voice methods and voice_item
   media builder for audio/.silk files.  Added video_md5 field.

3. Markdown links stripped — WeChat can't render [text](url), so
   format_message() now converts them to 'text (url)' plaintext.
   Code blocks are preserved.  (Fixes #7617)

4. Blank message prevention — three guards:
   a) _split_text_for_weixin_delivery('') returns [] not ['']
   b) send() filters empty/whitespace chunks before _send_text_chunk
   c) _send_message() raises ValueError for empty text as safety net

Community credit: joei4cm (#8407), lyonDan (#8521), SKFDJKLDG (#8360),
tomqiaozc (#7695), joshleeeeee (#8308), luoxiao6645(#8525),
longsizhuo (#7531), Astral-Yang (#8144), QingWei-Li (#8251).

tests/gateway/test_weixin.py

@@ -30,7 +30,7 @@ class TestWeixinFormatting:
 
         assert (
             adapter.format_message(content)
-            == "【Title】\n\n**Plan**\n\nUse **bold** and [docs](https://example.com)."
+            == "【Title】\n\n**Plan**\n\nUse **bold** and docs (https://example.com)."
         )
 
     def test_format_message_rewrites_markdown_tables(self):
@@ -374,3 +374,149 @@ class TestWeixinRemoteMediaSafety:
                 assert "Blocked unsafe URL" in str(exc)
             else:
                 raise AssertionError("expected ValueError for unsafe URL")
+
+
+class TestWeixinMarkdownLinks:
+    """Markdown links should be converted to plaintext since WeChat can't render them."""
+
+    def test_format_message_converts_markdown_links_to_plain_text(self):
+        adapter = _make_adapter()
+
+        content = "Check [the docs](https://example.com) and [GitHub](https://github.com) for details"
+        assert (
+            adapter.format_message(content)
+            == "Check the docs (https://example.com) and GitHub (https://github.com) for details"
+        )
+
+    def test_format_message_preserves_links_inside_code_blocks(self):
+        adapter = _make_adapter()
+
+        content = "See below:\n\n```\n[link](https://example.com)\n```\n\nDone."
+        result = adapter.format_message(content)
+        assert "[link](https://example.com)" in result
+
+
+class TestWeixinBlankMessagePrevention:
+    """Regression tests for the blank-bubble bugs.
+
+    Three separate guards now prevent a blank WeChat message from ever being
+    dispatched:
+
+    1. ``_split_text_for_weixin_delivery("")`` returns ``[]`` — not ``[""]``.
+    2. ``send()`` filters out empty/whitespace-only chunks before calling
+       ``_send_text_chunk``.
+    3. ``_send_message()`` raises ``ValueError`` for empty text as a last-resort
+       safety net.
+    """
+
+    def test_split_text_returns_empty_list_for_empty_string(self):
+        adapter = _make_adapter()
+        assert adapter._split_text("") == []
+
+    def test_split_text_returns_empty_list_for_empty_string_split_per_line(self):
+        adapter = WeixinAdapter(
+            PlatformConfig(
+                enabled=True,
+                extra={
+                    "account_id": "acct",
+                    "token": "test-tok",
+                    "split_multiline_messages": True,
+                },
+            )
+        )
+        assert adapter._split_text("") == []
+
+    @patch("gateway.platforms.weixin._send_message", new_callable=AsyncMock)
+    def test_send_empty_content_does_not_call_send_message(self, send_message_mock):
+        adapter = _make_adapter()
+        adapter._session = object()
+        adapter._token = "test-token"
+        adapter._base_url = "https://weixin.example.com"
+        adapter._token_store.get = lambda account_id, chat_id: "ctx-token"
+
+        result = asyncio.run(adapter.send("wxid_test123", ""))
+        # Empty content → no chunks → no _send_message calls
+        assert result.success is True
+        send_message_mock.assert_not_awaited()
+
+    def test_send_message_rejects_empty_text(self):
+        """_send_message raises ValueError for empty/whitespace text."""
+        import pytest
+        with pytest.raises(ValueError, match="text must not be empty"):
+            asyncio.run(
+                weixin._send_message(
+                    AsyncMock(),
+                    base_url="https://example.com",
+                    token="tok",
+                    to="wxid_test",
+                    text="",
+                    context_token=None,
+                    client_id="cid",
+                )
+            )
+
+
+class TestWeixinStreamingCursorSuppression:
+    """WeChat doesn't support message editing — cursor must be suppressed."""
+
+    def test_supports_message_editing_is_false(self):
+        adapter = _make_adapter()
+        assert adapter.SUPPORTS_MESSAGE_EDITING is False
+
+
+class TestWeixinMediaBuilder:
+    """Media builder uses base64(hex_key), not base64(raw_bytes) for aes_key."""
+
+    def test_image_builder_aes_key_is_base64_of_hex(self):
+        import base64
+        adapter = _make_adapter()
+        media_type, builder = adapter._outbound_media_builder("photo.jpg")
+        assert media_type == weixin.MEDIA_IMAGE
+
+        fake_hex_key = "0123456789abcdef0123456789abcdef"
+        expected_aes = base64.b64encode(fake_hex_key.encode("ascii")).decode("ascii")
+        item = builder(
+            encrypt_query_param="eq",
+            aes_key_for_api=expected_aes,
+            ciphertext_size=1024,
+            plaintext_size=1000,
+            filename="photo.jpg",
+            rawfilemd5="abc123",
+        )
+        assert item["image_item"]["media"]["aes_key"] == expected_aes
+
+    def test_video_builder_includes_md5(self):
+        adapter = _make_adapter()
+        media_type, builder = adapter._outbound_media_builder("clip.mp4")
+        assert media_type == weixin.MEDIA_VIDEO
+
+        item = builder(
+            encrypt_query_param="eq",
+            aes_key_for_api="fakekey",
+            ciphertext_size=2048,
+            plaintext_size=2000,
+            filename="clip.mp4",
+            rawfilemd5="deadbeef",
+        )
+        assert item["video_item"]["video_md5"] == "deadbeef"
+
+    def test_voice_builder_for_audio_files(self):
+        adapter = _make_adapter()
+        media_type, builder = adapter._outbound_media_builder("note.mp3")
+        assert media_type == weixin.MEDIA_VOICE
+
+        item = builder(
+            encrypt_query_param="eq",
+            aes_key_for_api="fakekey",
+            ciphertext_size=512,
+            plaintext_size=500,
+            filename="note.mp3",
+            rawfilemd5="abc",
+        )
+        assert item["type"] == weixin.ITEM_VOICE
+        assert "voice_item" in item
+
+    def test_voice_builder_for_silk_files(self):
+        adapter = _make_adapter()
+        media_type, builder = adapter._outbound_media_builder("recording.silk")
+        assert media_type == weixin.MEDIA_VOICE