fix(title-gen): surface auxiliary failures via _emit_auxiliary_failure

Closes #15775.

Title generation swallowed exceptions at debug level and returned None,
so a depleted auxiliary provider (e.g. OpenRouter 402) silently left
sessions with NULL titles. Reporter observed 45 untitled sessions
accumulated over 19 days with no user-visible indication.

- agent/title_generator.py: accept optional failure_callback, bump log
  to WARNING, invoke callback on call_llm exception (swallowing callback
  errors so nothing can crash the fire-and-forget worker thread).
- cli.py, gateway/run.py: pass agent._emit_auxiliary_failure as the
  callback so failures route through the existing user-visible warning
  channel.
- tests: cover callback fires / errors are swallowed / no-callback
  legacy behavior / maybe_auto_title forwards kwarg to worker.

cli.py

@@ -8672,12 +8672,20 @@ class HermesCLI:
             if response and result and not result.get("failed") and not result.get("partial"):
                 try:
                     from agent.title_generator import maybe_auto_title
+                    # Route title-generation failures through the agent's
+                    # user-visible warning channel so a depleted auxiliary
+                    # provider doesn't silently leave sessions untitled
+                    # (issue #15775).
+                    _title_failure_cb = getattr(
+                        self.agent, "_emit_auxiliary_failure", None
+                    ) if self.agent else None
                     maybe_auto_title(
                         self._session_db,
                         self.session_id,
                         message,
                         response,
                         self.conversation_history,
+                        failure_callback=_title_failure_cb,
                     )
                 except Exception:
                     pass