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.

agent/context_compressor.py

@@ -340,6 +340,8 @@ class ContextCompressor(ContextEngine):
         self._last_summary_error = None
         self._last_summary_dropped_count = 0
         self._last_summary_fallback_used = False
+        self._last_aux_model_failure_error = None
+        self._last_aux_model_failure_model = None
         self._last_compression_savings_pct = 100.0
         self._ineffective_compression_count = 0
 
@@ -448,6 +450,12 @@ class ContextCompressor(ContextEngine):
         # (gateway hygiene, /compress) can surface a visible warning.
         self._last_summary_dropped_count: int = 0
         self._last_summary_fallback_used: bool = False
+        # When a user-configured summary model fails and we recover by
+        # retrying on the main model, record the failure so gateway /
+        # CLI callers can still warn the user even though compression
+        # succeeded.  Silent recovery would hide the broken config.
+        self._last_aux_model_failure_error: Optional[str] = None
+        self._last_aux_model_failure_model: Optional[str] = None
 
     def update_from_response(self, usage: Dict[str, Any]):
         """Update tracked token usage from API response."""
@@ -907,6 +915,14 @@ The user has requested that this compaction PRIORITISE preserving all informatio
                     "Falling back to main model '%s' for compression.",
                     self.summary_model, e, self.model,
                 )
+                # Record the aux-model failure so callers can warn the user
+                # even if the retry-on-main succeeds — a misconfigured aux
+                # model is something the user needs to fix.
+                _err_text = str(e).strip() or e.__class__.__name__
+                if len(_err_text) > 220:
+                    _err_text = _err_text[:217].rstrip() + "..."
+                self._last_aux_model_failure_error = _err_text
+                self._last_aux_model_failure_model = self.summary_model
                 self.summary_model = ""  # empty = use main model
                 self._summary_failure_cooldown_until = 0.0  # no cooldown
                 return self._generate_summary(turns_to_summarize, focus_topic=focus_topic)  # retry immediately
@@ -931,6 +947,14 @@ The user has requested that this compaction PRIORITISE preserving all informatio
                     "Retrying on main model '%s' before giving up.",
                     self.summary_model, e, self.model,
                 )
+                # Record the aux-model failure (see 404 branch above) — user
+                # should know their configured model is broken even if main
+                # recovers the call.
+                _err_text = str(e).strip() or e.__class__.__name__
+                if len(_err_text) > 220:
+                    _err_text = _err_text[:217].rstrip() + "..."
+                self._last_aux_model_failure_error = _err_text
+                self._last_aux_model_failure_model = self.summary_model
                 self.summary_model = ""  # empty = use main model
                 self._summary_failure_cooldown_until = 0.0
                 return self._generate_summary(turns_to_summarize, focus_topic=focus_topic)
@@ -1232,6 +1256,8 @@ The user has requested that this compaction PRIORITISE preserving all informatio
         self._last_summary_dropped_count = 0
         self._last_summary_fallback_used = False
         self._last_summary_error = None
+        self._last_aux_model_failure_error = None
+        self._last_aux_model_failure_model = None
         n_messages = len(messages)
         # Only need head + 3 tail messages minimum (token budget decides the real tail size)
         _min_for_compress = self.protect_first_n + 3 + 1