fix(error_classifier): handle dict-typed message fields without crashing

When API providers return Pydantic-style validation errors where
body['message'] or body['error']['message'] is a dict (e.g.
{"detail": [...]}), the error classifier was crashing with
AttributeError: 'dict' object has no attribute 'lower'.

The 'or ""' fallback only handles None/falsy values. A non-empty
dict is truthy and passes through to .lower(), which fails.

Fix: Wrap all 5 call sites with str() before calling .lower().
This is a no-op for strings and safely converts dicts to their
repr for pattern matching (no false positives on classification
patterns like 'rate limit', 'context length', etc.).

Closes #11233

tests/agent/test_error_classifier.py

@@ -849,3 +849,73 @@ class TestAdversarialEdgeCases:
         )
         result = classify_api_error(e, provider="openrouter")
         assert result.reason == FailoverReason.model_not_found
+
+    # ── Regression: dict-typed message field (Issue #11233) ──
+
+    def test_pydantic_dict_message_no_crash(self):
+        """Pydantic validation errors return message as dict, not string.
+
+        Regression: classify_api_error must not crash when body['message']
+        is a dict (e.g. {"detail": [...]} from FastAPI/Pydantic). The
+        'or ""' fallback only handles None/falsy values — a non-empty
+        dict is truthy and passed to .lower(), causing AttributeError.
+        """
+        e = MockAPIError(
+            "Unprocessable Entity",
+            status_code=422,
+            body={
+                "object": "error",
+                "message": {
+                    "detail": [
+                        {
+                            "type": "extra_forbidden",
+                            "loc": ["body", "think"],
+                            "msg": "Extra inputs are not permitted",
+                        }
+                    ]
+                },
+            },
+        )
+        result = classify_api_error(e)
+        assert result.reason == FailoverReason.format_error
+        assert result.status_code == 422
+        assert result.retryable is False
+
+    def test_nested_error_dict_message_no_crash(self):
+        """Nested body['error']['message'] as dict must not crash.
+
+        Some providers wrap Pydantic errors in an 'error' object.
+        """
+        e = MockAPIError(
+            "Validation error",
+            status_code=400,
+            body={
+                "error": {
+                    "message": {
+                        "detail": [
+                            {"type": "missing", "loc": ["body", "required"]}
+                        ]
+                    }
+                }
+            },
+        )
+        result = classify_api_error(e, approx_tokens=1000)
+        assert result.reason == FailoverReason.format_error
+        assert result.status_code == 400
+
+    def test_metadata_raw_dict_message_no_crash(self):
+        """OpenRouter metadata.raw with dict message must not crash."""
+        e = MockAPIError(
+            "Provider error",
+            status_code=400,
+            body={
+                "error": {
+                    "message": "Provider error",
+                    "metadata": {
+                        "raw": '{"error":{"message":{"detail":[{"type":"invalid"}]}}}'
+                    }
+                }
+            },
+        )
+        result = classify_api_error(e)
+        assert result.reason == FailoverReason.format_error