fix(gateway): mark only still-running sessions resume_pending on drain timeout (#12332)

Follow-up to #12301.

The drain-timeout branch of _stop_impl() was iterating the drain-start
snapshot (active_agents) when marking sessions resume_pending. That
snapshot can include sessions that finished gracefully during the drain
window — marking them would give their next turn a stray
'your previous turn was interrupted by a gateway restart' system note
even though the prior turn actually completed cleanly.

Iterate self._running_agents at timeout time instead, mirroring
_interrupt_running_agents() exactly:
- only sessions still blocking the shutdown get marked
- pending sentinels (AIAgent construction not yet complete) are skipped

Changes:
- gateway/run.py: swap active_agents.keys() for filtered
  self._running_agents.items() iteration in the drain-timeout mark loop.
- tests/gateway/test_restart_resume_pending.py: two regression tests —
  finisher-during-drain not marked, pending sentinel not marked.

gateway/run.py

@@ -2383,10 +2383,23 @@ class GatewayRunner:
                 # existing ``.restart_failure_counts`` stuck-loop counter
                 # (incremented below, threshold 3), which sets
                 # ``suspended=True`` and overrides resume_pending.
+                #
+                # Iterate self._running_agents (current) rather than the
+                # drain-start ``active_agents`` snapshot — the snapshot
+                # may include sessions that finished gracefully during
+                # the drain window, and marking those falsely would give
+                # them a stray restart-interruption system note on their
+                # next turn even though their previous turn completed
+                # cleanly.  Skip pending sentinels for the same reason
+                # _interrupt_running_agents() does: their agent hasn't
+                # started yet, there's nothing to interrupt, and the
+                # session shouldn't carry a misleading resume flag.
                 _resume_reason = (
                     "restart_timeout" if self._restart_requested else "shutdown_timeout"
                 )
-                for _sk in list(active_agents.keys()):
+                for _sk, _agent in list(self._running_agents.items()):
+                    if _agent is _AGENT_PENDING_SENTINEL:
+                        continue
                     try:
                         self.session_store.mark_resume_pending(_sk, _resume_reason)
                     except Exception as _e: