fix(testing): improve api_recorder error messages for missing recordings (#3760)

Replaces opaque error messages when recordings are not found with
somewhat better guidance

Before:
```
No recorded response found for request hash: abc123...
To record this response, run with LLAMA_STACK_TEST_INFERENCE_MODE=record
```

After:
```
Recording not found for request hash: abc123
Model: gpt-4 | Request: POST https://api.openai.com/v1/chat/completions

Run './scripts/integration-tests.sh --inference-mode record-if-missing' with required API keys to generate.
```

tests/unit/distribution/test_api_recordings.py

@@ -261,7 +261,7 @@ class TestInferenceRecording:
             with api_recording(mode=APIRecordingMode.REPLAY, storage_dir=str(temp_storage_dir)):
                 client = AsyncOpenAI(base_url="http://localhost:11434/v1", api_key="test")
 
-                with pytest.raises(RuntimeError, match="No recorded response found"):
+                with pytest.raises(RuntimeError, match="Recording not found"):
                     await client.chat.completions.create(
                         model="llama3.2:3b", messages=[{"role": "user", "content": "This was never recorded"}]
                     )