fix(error_classifier): don't classify generic 404 as model_not_found (#14013)

The 404 branch in _classify_by_status had dead code: the generic
fallback below the _MODEL_NOT_FOUND_PATTERNS check returned the
exact same classification (model_not_found + should_fallback=True),
so every 404 — regardless of message — was treated as a missing model.

This bites local-endpoint users (llama.cpp, Ollama, vLLM) whose 404s
usually mean a wrong endpoint path, proxy routing glitch, or transient
backend issue — not a missing model. Claiming 'model not found' misleads
the next turn and silently falls back to another provider when the real
problem was a URL typo the user should see.

Fix: only classify 404 as model_not_found when the message actually
matches _MODEL_NOT_FOUND_PATTERNS ("invalid model", "model not found",
etc.). Otherwise fall through as unknown (retryable) so the real error
surfaces in the retry loop.

Test updated to match the new behavior. 103 error_classifier tests pass.

tests/agent/test_error_classifier.py

@@ -298,9 +298,15 @@ class TestClassifyApiError:
         assert result.retryable is False
 
     def test_404_generic(self):
+        # Generic 404 with no "model not found" signal — common for local
+        # llama.cpp/Ollama/vLLM endpoints with slightly wrong paths.  Treat
+        # as unknown (retryable) so the real error surfaces, rather than
+        # claiming the model is missing and silently falling back.
         e = MockAPIError("Not Found", status_code=404)
         result = classify_api_error(e)
-        assert result.reason == FailoverReason.model_not_found
+        assert result.reason == FailoverReason.unknown
+        assert result.retryable is True
+        assert result.should_fallback is False
 
     # ── Payload too large ──