fix(gateway): preserve sender attribution in shared group sessions

Generalize shared multi-user session handling so non-thread group sessions
(group_sessions_per_user=False) get the same treatment as shared threads:
inbound messages are prefixed with [sender name], and the session prompt
shows a multi-user note instead of pinning a single **User:** line into
the cached system prompt.

Before: build_session_key already treated these as shared sessions, but
_prepare_inbound_message_text and build_session_context_prompt only
recognized shared threads — creating cross-user attribution drift and
prompt-cache contamination in shared groups.

- Add is_shared_multi_user_session() helper alongside build_session_key()
  so both the session key and the multi-user branches are driven by the
  same rules (DMs never shared, threads shared unless
  thread_sessions_per_user, groups shared unless group_sessions_per_user).
- Add shared_multi_user_session field to SessionContext, populated by
  build_session_context() from config.
- Use context.shared_multi_user_session in the prompt builder (label is
  'Multi-user thread' when a thread is present, 'Multi-user session'
  otherwise).
- Use the helper in _prepare_inbound_message_text so non-thread shared
  groups also get [sender] prefixes.

Default behavior unchanged: DMs stay single-user, groups with
group_sessions_per_user=True still show the user normally, shared threads
keep their existing multi-user behavior.

Tests (65 passed):
- tests/gateway/test_session.py: new shared non-thread group prompt case.
- tests/gateway/test_shared_group_sender_prefix.py: inbound preprocessing
  for shared non-thread groups and default groups.

tests/gateway/test_session.py

@@ -356,6 +356,28 @@ class TestBuildSessionContextPrompt:
         assert "**User:** Alice" in prompt
         assert "Multi-user thread" not in prompt
 
+    def test_shared_non_thread_group_prompt_hides_single_user(self):
+        """Shared non-thread group sessions should avoid pinning one user."""
+        config = GatewayConfig(
+            platforms={
+                Platform.TELEGRAM: PlatformConfig(enabled=True, token="fake"),
+            },
+            group_sessions_per_user=False,
+        )
+        source = SessionSource(
+            platform=Platform.TELEGRAM,
+            chat_id="-1002285219667",
+            chat_name="Test Group",
+            chat_type="group",
+            user_name="Alice",
+        )
+        ctx = build_session_context(source, config)
+        prompt = build_session_context_prompt(ctx)
+
+        assert "Multi-user session" in prompt
+        assert "[sender name]" in prompt
+        assert "**User:** Alice" not in prompt
+
     def test_dm_thread_shows_user_not_multi(self):
         """DM threads are single-user and should show User, not multi-user note."""
         config = GatewayConfig(

tests/gateway/test_shared_group_sender_prefix.py

@@ -0,0 +1,70 @@
+import pytest
+
+from gateway.config import GatewayConfig, Platform, PlatformConfig
+from gateway.platforms.base import MessageEvent
+from gateway.run import GatewayRunner
+from gateway.session import SessionSource
+
+
+def _make_runner(config: GatewayConfig) -> GatewayRunner:
+    runner = object.__new__(GatewayRunner)
+    runner.config = config
+    runner.adapters = {}
+    runner._model = "openai/gpt-4.1-mini"
+    runner._base_url = None
+    return runner
+
+
+@pytest.mark.asyncio
+async def test_preprocess_prefixes_sender_for_shared_non_thread_group_session():
+    runner = _make_runner(
+        GatewayConfig(
+            platforms={
+                Platform.TELEGRAM: PlatformConfig(enabled=True, token="fake"),
+            },
+            group_sessions_per_user=False,
+        )
+    )
+    source = SessionSource(
+        platform=Platform.TELEGRAM,
+        chat_id="-1002285219667",
+        chat_name="Test Group",
+        chat_type="group",
+        user_name="Alice",
+    )
+    event = MessageEvent(text="hello", source=source)
+
+    result = await runner._prepare_inbound_message_text(
+        event=event,
+        source=source,
+        history=[],
+    )
+
+    assert result == "[Alice] hello"
+
+
+@pytest.mark.asyncio
+async def test_preprocess_keeps_plain_text_for_default_group_sessions():
+    runner = _make_runner(
+        GatewayConfig(
+            platforms={
+                Platform.TELEGRAM: PlatformConfig(enabled=True, token="fake"),
+            },
+        )
+    )
+    source = SessionSource(
+        platform=Platform.TELEGRAM,
+        chat_id="-1002285219667",
+        chat_name="Test Group",
+        chat_type="group",
+        user_name="Alice",
+    )
+    event = MessageEvent(text="hello", source=source)
+
+    result = await runner._prepare_inbound_message_text(
+        event=event,
+        source=source,
+        history=[],
+    )
+
+    assert result == "hello"