refactor(gateway): simplify auto-resume + extend to crash recovery

Follow-up on top of @kyan12's PR #20888 — same feature, cleaner shape,
wider coverage.

Changes:
- Drop the synthetic '[System note: ...]' in the internal MessageEvent.
  The existing _is_resume_pending branch in _handle_message_with_agent
  (run.py ~L13738) already injects a reason-aware recovery system note
  on the next turn.  With kyan's text in place the model saw two stacked
  system notes.  Now the event text is empty and the existing injection
  path owns the wording.
- Drop SessionStore.list_resume_pending() as a new public method.  The
  filter is 8 lines inline in _schedule_resume_pending_sessions() —
  one caller, no other pluggability need.
- Add 'restart_interrupted' to the auto-resume reason set.  That's the
  reason SessionStore.suspend_recently_active() stamps on sessions
  recovered from a crash/OOM/SIGKILL (no .clean_shutdown marker).
  Previously those sessions had to wait for a real user message to
  auto-resume; now they continue automatically at startup like
  drain-timeout interruptions do.
- Reasons live in a _AUTO_RESUME_REASONS frozenset at class scope so
  future reasons (e.g. 'manual_resume_request') can be opted in with
  one line.

Test coverage added:
- drain-timeout + crash-recovery both scheduled
- stale entries skipped (outside freshness window)
- suspended entries skipped (suspended > resume_pending)
- originless entries skipped (no routing target)
- disallowed reasons skipped (graceful forward-compat)

E2E verified end-to-end with a real on-disk SessionStore: 2 eligible
sessions scheduled, 2 ineligible skipped, empty-text internal events
delivered to the adapter.

Co-authored-by: Kevin Yan <kevyan1998@gmail.com>

gateway/run.py

@@ -2759,44 +2759,67 @@ class GatewayRunner:
         task.add_done_callback(self._background_tasks.discard)
         return True
 
+    # Drain-timeout reasons set by _stop_impl() when a still-running turn is
+    # force-interrupted; "restart_interrupted" is set by
+    # SessionStore.suspend_recently_active() on crash recovery (no
+    # .clean_shutdown marker).  All three mean "the agent was mid-turn and
+    # we killed it" — eligible for startup auto-resume.
+    _AUTO_RESUME_REASONS = frozenset(
+        {"restart_timeout", "shutdown_timeout", "restart_interrupted"}
+    )
+
     def _schedule_resume_pending_sessions(self) -> int:
         """Auto-continue fresh restart-interrupted sessions after startup.
 
-        ``resume_pending`` already preserves the transcript and injects the
-        recovery system note on the next user message.  This method closes the
-        restart UX gap by synthesizing that next message once adapters are back
-        online, so users do not have to send a placeholder ping after restart.
+        ``resume_pending`` already preserves the transcript AND the existing
+        ``_is_resume_pending`` branch in ``_handle_message_with_agent``
+        injects a reason-aware recovery system note on the next turn.  This
+        method closes the UX gap by synthesizing that next turn once
+        adapters are back online — the event text is empty so the existing
+        injection path owns the wording and we never double up.
+
+        Adapters that are not yet ready (adapter missing from
+        ``self.adapters``) are skipped silently; their sessions stay
+        ``resume_pending`` and will auto-resume on the next real user
+        message, or on the next gateway startup.
         """
+        window = _auto_continue_freshness_window()
         try:
-            entries = self.session_store.list_resume_pending(
-                window_secs=_auto_continue_freshness_window(),
-                allowed_reasons={"restart_timeout", "shutdown_timeout"},
-            )
+            with self.session_store._lock:  # noqa: SLF001 — snapshot under lock
+                self.session_store._ensure_loaded_locked()  # noqa: SLF001
+                candidates = [
+                    entry for entry in self.session_store._entries.values()  # noqa: SLF001
+                    if entry.resume_pending
+                    and not entry.suspended
+                    and entry.origin is not None
+                    and entry.resume_reason in self._AUTO_RESUME_REASONS
+                ]
         except Exception as exc:
-            logger.warning("Failed to list resume-pending sessions: %s", exc)
+            logger.warning("Failed to enumerate resume-pending sessions: %s", exc)
             return 0
 
+        now = datetime.now()
         scheduled = 0
-        for entry in entries:
-            source = getattr(entry, "origin", None)
-            platform = getattr(source, "platform", None)
-            adapter = self.adapters.get(platform) if platform is not None else None
-            if source is None or adapter is None:
+        for entry in candidates:
+            marker = entry.last_resume_marked_at or entry.updated_at
+            if marker is not None and (now - marker).total_seconds() > window:
+                continue
+
+            source = entry.origin
+            adapter = self.adapters.get(source.platform)
+            if adapter is None:
                 logger.debug(
-                    "Skipping auto-resume for %s: adapter unavailable for %s",
-                    getattr(entry, "session_key", "?"),
-                    getattr(platform, "value", platform),
+                    "Skipping auto-resume for %s: adapter not ready for %s",
+                    entry.session_key,
+                    getattr(source.platform, "value", source.platform),
                 )
                 continue
 
+            # Empty-text internal event — the _is_resume_pending branch in
+            # _handle_message_with_agent prepends the proper reason-aware
+            # system note before the turn runs.
             event = MessageEvent(
-                text=(
-                    "[System note: The gateway restarted after interrupting "
-                    "this session. Resume the previous turn now. Reconcile "
-                    "the transcript first: if tool results are already present, "
-                    "process them before taking new action; never claim work "
-                    "completed unless it is visible in the transcript/tool output.]"
-                ),
+                text="",
                 message_type=MessageType.TEXT,
                 source=source,
                 internal=True,
@@ -2807,7 +2830,10 @@ class GatewayRunner:
             scheduled += 1
 
         if scheduled:
-            logger.info("Scheduled auto-resume for %d restart-interrupted session(s)", scheduled)
+            logger.info(
+                "Scheduled auto-resume for %d restart-interrupted session(s)",
+                scheduled,
+            )
         return scheduled
 
     async def start(self) -> bool:

gateway/session.py

@@ -1028,42 +1028,6 @@ class SessionStore:
             self._save()
             return True
 
-    def list_resume_pending(
-        self,
-        *,
-        window_secs: Optional[float] = None,
-        now: Optional[float] = None,
-        allowed_reasons: Optional[set[str]] = None,
-    ) -> List[SessionEntry]:
-        """Return fresh restart-interrupted sessions eligible for resume.
-
-        Only entries that still have an origin are returned; the gateway needs
-        that origin to route continuation back through the original
-        platform/chat/thread.  ``suspended`` entries are excluded because
-        explicit suspension/stuck-loop escalation must win over resume.
-        """
-        current = datetime.fromtimestamp(now) if now is not None else _now()
-        window = float(window_secs) if window_secs is not None else None
-
-        with self._lock:
-            self._ensure_loaded_locked()
-            entries = list(self._entries.values())
-
-        pending: List[SessionEntry] = []
-        for entry in entries:
-            if not entry.resume_pending or entry.suspended or entry.origin is None:
-                continue
-            if allowed_reasons is not None and entry.resume_reason not in allowed_reasons:
-                continue
-            if window is not None and window > 0:
-                marker = entry.last_resume_marked_at or entry.updated_at
-                if marker is not None and (current - marker).total_seconds() > window:
-                    continue
-            pending.append(entry)
-
-        pending.sort(key=lambda entry: entry.last_resume_marked_at or entry.updated_at)
-        return pending
-
     def prune_old_entries(self, max_age_days: int) -> int:
         """Drop SessionEntry records older than max_age_days.