Add changelog for #4389

changelog/4389.fixed.md

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+- Fixed AWS services failing silently on missing or invalid credentials.
+  `AWSNovaSonicLLMService`, `AWSBedrockLLMService`, `AWSPollyTTSService`,
+  and `AWSTranscribeSTTService` now push a fatal `ErrorFrame` with a
+  "check AWS credentials and region" hint on auth-class failures, so the
+  pipeline cancels promptly instead of continuing to run with no output.
+- Fixed `AWSNovaSonicLLMService._disconnect` raising `InvalidStateError`
+  from `awscrt/aio/http.py` when cleanup ran on a stream from a failed
+  `invoke_model_with_bidirectional_stream` call. The error was masking
+  the real connect-time auth failure in the logs.