fix(kanban): auto-block workers that exit without completing (#20894) (#21214)

When a kanban worker subprocess exits rc=0 but its task is still in
status='running', the agent almost certainly answered the task
conversationally without calling kanban_complete or kanban_block. The
dispatcher used to classify this as a generic crash and respawn, which
loops forever on small local models (gemma4-e2b q4 etc.) that keep
returning clean but unproductive output.

Dispatcher changes:
- The waitpid reap loop at the top of dispatch_once now records each
  reaped child's raw exit status in a bounded module registry
  (_recent_worker_exits, TTL 600s, size cap 4096).
- _classify_worker_exit distinguishes clean_exit / nonzero_exit /
  signaled / unknown using os.WIFEXITED / WIFSIGNALED.
- detect_crashed_workers consults the classification when a worker
  is found dead. clean_exit → protocol_violation event + immediate
  circuit-breaker trip (failure_limit=1). Everything else keeps the
  existing crashed-event + counter behavior.
- DispatchResult.auto_blocked now includes protocol-violation trips.

Gateway fix (Bug A in #20894):
- gateway.run._notify_active_sessions_of_shutdown snapshots
  self.adapters with list(...) before iterating. adapter.send() can
  hit a fatal-error path that pops the adapter from the dict, which
  was raising 'RuntimeError: dictionary changed size during iteration'
  during shutdown.

Regression tests:
- test_detect_crashed_workers_protocol_violation_auto_blocks verifies
  rc=0 + still-running → status=blocked on first occurrence with
  protocol_violation + gave_up events and NO crashed event.
- test_detect_crashed_workers_nonzero_exit_uses_default_limit verifies
  non-zero exits keep the existing 2-strike behavior.

Closes #20894.

gateway/run.py

@@ -2521,7 +2521,12 @@ class GatewayRunner:
                     platform_str, chat_id, e,
                 )
 
-        for platform, adapter in self.adapters.items():
+        # Snapshot adapters up front: adapter.send() can hit a fatal error
+        # path that pops the adapter from self.adapters (see _handle_fatal
+        # elsewhere), which would otherwise trigger
+        # ``RuntimeError: dictionary changed size during iteration`` —
+        # observed in a user report during gateway shutdown.
+        for platform, adapter in list(self.adapters.items()):
             home = self.config.get_home_channel(platform)
             if not home or not home.chat_id:
                 continue