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.

gateway/run.py

@@ -279,6 +279,7 @@ from gateway.session import (
     build_session_context,
     build_session_context_prompt,
     build_session_key,
+    is_shared_multi_user_session,
 )
 from gateway.delivery import DeliveryRouter
 from gateway.platforms.base import (
@@ -3789,12 +3790,12 @@ class GatewayRunner:
         history = history or []
         message_text = event.text or ""
 
-        _is_shared_thread = (
-            source.chat_type != "dm"
-            and source.thread_id
-            and not getattr(self.config, "thread_sessions_per_user", False)
+        _is_shared_multi_user = is_shared_multi_user_session(
+            source,
+            group_sessions_per_user=getattr(self.config, "group_sessions_per_user", True),
+            thread_sessions_per_user=getattr(self.config, "thread_sessions_per_user", False),
         )
-        if _is_shared_thread and source.user_name:
+        if _is_shared_multi_user and source.user_name:
             message_text = f"[{source.user_name}] {message_text}"
 
         if event.media_urls:

gateway/session.py

@@ -152,6 +152,7 @@ class SessionContext:
     source: SessionSource
     connected_platforms: List[Platform]
     home_channels: Dict[Platform, HomeChannel]
+    shared_multi_user_session: bool = False
     
     # Session metadata
     session_key: str = ""
@@ -166,6 +167,7 @@ class SessionContext:
             "home_channels": {
                 p.value: hc.to_dict() for p, hc in self.home_channels.items()
             },
+            "shared_multi_user_session": self.shared_multi_user_session,
             "session_key": self.session_key,
             "session_id": self.session_id,
             "created_at": self.created_at.isoformat() if self.created_at else None,
@@ -240,18 +242,16 @@ def build_session_context_prompt(
         lines.append(f"**Channel Topic:** {context.source.chat_topic}")
 
     # User identity.
-    # In shared thread sessions (non-DM with thread_id), multiple users
-    # contribute to the same conversation.  Don't pin a single user name
-    # in the system prompt — it changes per-turn and would bust the prompt
-    # cache.  Instead, note that this is a multi-user thread; individual
-    # sender names are prefixed on each user message by the gateway.
-    _is_shared_thread = (
-        context.source.chat_type != "dm"
-        and context.source.thread_id
-    )
-    if _is_shared_thread:
+    # In shared multi-user sessions (shared threads OR shared non-thread groups
+    # when group_sessions_per_user=False), multiple users contribute to the same
+    # conversation.  Don't pin a single user name in the system prompt — it
+    # changes per-turn and would bust the prompt cache.  Instead, note that
+    # this is a multi-user session; individual sender names are prefixed on
+    # each user message by the gateway.
+    if context.shared_multi_user_session:
+        session_label = "Multi-user thread" if context.source.thread_id else "Multi-user session"
         lines.append(
-            "**Session type:** Multi-user thread — messages are prefixed "
+            f"**Session type:** {session_label} — messages are prefixed "
             "with [sender name]. Multiple users may participate."
         )
     elif context.source.user_name:
@@ -467,6 +467,27 @@ class SessionEntry:
         )
 
 
+def is_shared_multi_user_session(
+    source: SessionSource,
+    *,
+    group_sessions_per_user: bool = True,
+    thread_sessions_per_user: bool = False,
+) -> bool:
+    """Return True when a non-DM session is shared across participants.
+
+    Mirrors the isolation rules in :func:`build_session_key`:
+      - DMs are never shared.
+      - Threads are shared unless ``thread_sessions_per_user`` is True.
+      - Non-thread group/channel sessions are shared unless
+        ``group_sessions_per_user`` is True (default: True = isolated).
+    """
+    if source.chat_type == "dm":
+        return False
+    if source.thread_id:
+        return not thread_sessions_per_user
+    return not group_sessions_per_user
+
+
 def build_session_key(
     source: SessionSource,
     group_sessions_per_user: bool = True,
@@ -1238,6 +1259,11 @@ def build_session_context(
         source=source,
         connected_platforms=connected,
         home_channels=home_channels,
+        shared_multi_user_session=is_shared_multi_user_session(
+            source,
+            group_sessions_per_user=getattr(config, "group_sessions_per_user", True),
+            thread_sessions_per_user=getattr(config, "thread_sessions_per_user", False),
+        ),
     )
     
     if session_entry: