fix(compression): notify users when configured aux model fails even if main-model fallback recovers (#16775)

A misconfigured auxiliary.compression.model is a user-fixable problem that silent recovery would hide. The previous retry-on-main logic transparently swallowed aux-model failures whenever the fallback succeeded, leaving the user's broken config in place and racking up future failures.

Track the aux-model failure on the compressor alongside the existing fallback-placeholder fields:
- _last_aux_model_failure_model: str | None
- _last_aux_model_failure_error: str | None

Both are set at the moment the aux model errors (captured before summary_model is cleared for retry), regardless of whether the retry succeeds. Cleared at compress() start and on on_session_reset() so a clean run doesn't leak stale warnings.

Surface at three places:
- gateway hygiene auto-compress: ℹ note to the platform adapter (thread_id preserved)
- gateway /compress command: ℹ line appended to the reply
- CLI via _emit_warning: deduped on (model, error) so repeat compactions don't spam

Distinct from the existing ⚠️ dropped-turns warning — different severity, different emoji, explicit 'context is intact' reassurance.

run_agent.py

@@ -8460,6 +8460,23 @@ class AIAgent:
                     f"⚠ Compression summary failed: {summary_error}. "
                     "Inserted a fallback context marker."
                 )
+        else:
+            # No hard failure — but did the configured aux model error out
+            # and get recovered by retrying on main?  Surface that so users
+            # know their auxiliary.compression.model setting is broken even
+            # though compression succeeded.
+            _aux_fail_model = getattr(self.context_compressor, "_last_aux_model_failure_model", None)
+            _aux_fail_err = getattr(self.context_compressor, "_last_aux_model_failure_error", None)
+            if _aux_fail_model:
+                # Dedup on (model, error) so we don't spam on every compaction
+                _aux_key = (_aux_fail_model, _aux_fail_err)
+                if getattr(self, "_last_aux_fallback_warning_key", None) != _aux_key:
+                    self._last_aux_fallback_warning_key = _aux_key
+                    self._emit_warning(
+                        f"ℹ Configured compression model '{_aux_fail_model}' failed "
+                        f"({_aux_fail_err or 'unknown error'}). Recovered using main model — "
+                        "check auxiliary.compression.model in config.yaml."
+                    )
 
         todo_snapshot = self._todo_store.format_for_injection()
         if todo_snapshot: