fix(bedrock): guard _chunk_bytes_to_sse against None or non-bytes payloads (#1975)

[iosayin]

Summary

Guards _chunk_bytes_to_sse in anthropic.lib.bedrock._stream_decoder against None or non-bytes payloads.

When AWS Bedrock sends an empty chunk frame, a control payload, or a chunk dictionary where chunk.get("bytes") is None (e.g. chunk = {"bytes": None} or chunk = {}), _chunk_bytes_to_sse previously called raw.decode() unconditionally, resulting in AttributeError: 'NoneType' object has no attribute 'decode'.

With this change, _chunk_bytes_to_sse safely returns None to drop non-bytes or empty payloads without crashing the active stream.

Closes #1975

[chauvuusvn]

Note that PR #1976 was already submitted on Oct 5 with regression test coverage to resolve #1975 prior to this PR. Please coordinate or consolidate review on #1976 to avoid duplicate PR overhead for the maintainers.

[iosayin]

Thanks for flagging, @chauvuusvn. Closing in favor of #1976, which was opened earlier for #1975 and already includes a regression test.