fix(gateway): cap cached session sources with LRU eviction

Follow-up on top of Zyproth's session-source cache: swap the unbounded
dict for an OrderedDict with a 512-entry LRU cap so long-running
gateways can't accumulate stale entries for dead sessions forever.

- self._session_sources is now an OrderedDict
- _cache_session_source() move_to_end + popitem(last=False) above cap
- _get_cached_session_source() move_to_end on hit (LRU read bump)
- restart_test_helpers.py wires OrderedDict + _session_sources_max

gateway/run.py

@@ -1086,7 +1086,13 @@ class GatewayRunner:
         self._pending_native_image_paths_by_session: Dict[str, List[str]] = {}
         self._busy_ack_ts: Dict[str, float] = {}  # last busy-ack timestamp per session (debounce)
         self._session_run_generation: Dict[str, int] = {}
-        self._session_sources: Dict[str, "SessionSource"] = {}
+        # LRU cache of live SessionSources keyed by session_key. Used by
+        # fallback routing paths (shutdown notifications, synthetic
+        # background-process events) when the persisted origin is missing
+        # and _parse_session_key can't recover thread_id. Capped so it
+        # cannot grow unbounded over a long-running gateway lifetime.
+        self._session_sources: "OrderedDict[str, SessionSource]" = OrderedDict()
+        self._session_sources_max = 512
 
         # Cache AIAgent instances per session to preserve prompt caching.
         # Without this, a new AIAgent is created per message, rebuilding the
@@ -6015,12 +6021,21 @@ class GatewayRunner:
             return
         cached_sources = getattr(self, "_session_sources", None)
         if cached_sources is None:
-            cached_sources = {}
+            cached_sources = OrderedDict()
             self._session_sources = cached_sources
         try:
             cached_sources[session_key] = dataclasses.replace(source)
         except Exception:
             logger.debug("Failed to cache live session source for %s", session_key, exc_info=True)
+            return
+        # LRU: mark as most-recently-used and trim to max size.
+        try:
+            cached_sources.move_to_end(session_key)
+            max_size = getattr(self, "_session_sources_max", 512)
+            while len(cached_sources) > max_size:
+                cached_sources.popitem(last=False)
+        except Exception:
+            pass
 
     def _get_cached_session_source(self, session_key: str):
         if not session_key:
@@ -6028,7 +6043,13 @@ class GatewayRunner:
         cached_sources = getattr(self, "_session_sources", None)
         if not cached_sources:
             return None
-        return cached_sources.get(session_key)
+        source = cached_sources.get(session_key)
+        if source is not None:
+            try:
+                cached_sources.move_to_end(session_key)
+            except Exception:
+                pass
+        return source
 
     async def _handle_message_with_agent(self, event, source, _quick_key: str, run_generation: int):
         """Inner handler that runs under the _running_agents sentinel guard."""