fix(gateway): /stop no longer resets the session (#9224)

/stop was calling suspend_session() which marked the session for auto-reset
on the next message. This meant users lost their conversation history every
time they stopped a running agent — especially painful for untitled sessions
that can't be resumed by name.

Now /stop just interrupts the agent and cleans the session lock. The session
stays intact so users can continue the conversation.

The suspend behavior was introduced in #7536 to break stuck session resume
loops on gateway restart. That case is already handled by
suspend_recently_active() which runs at gateway startup, so removing it from
/stop doesn't regress the original fix.

tests/gateway/test_session_race_guard.py

@@ -242,9 +242,7 @@ async def test_stop_during_sentinel_force_cleans_session():
         stop_event = _make_event(text="/stop")
         result = await runner._handle_message(stop_event)
         assert result is not None, "/stop during sentinel should return a message"
-        assert "force-stopped" in result.lower() or "unlocked" in result.lower()
-
-        # Sentinel must be cleaned up
+        assert "stopped" in result.lower()
         assert session_key not in runner._running_agents, (
             "/stop must remove sentinel so the session is unlocked"
         )
@@ -291,7 +289,7 @@ async def test_stop_hard_kills_running_agent():
 
     # Must return a confirmation
     assert result is not None
-    assert "force-stopped" in result.lower() or "unlocked" in result.lower()
+    assert "stopped" in result.lower()
 
 
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