fix(bigtable): preserve row key across ReadRowsResponse boundaries in chunk_stream

[mutianf]

Summary

• current_key was reset to None at the start of each ReadRowsResponse inside chunk_stream. When a single row spans two response messages, the continuation chunk in the second response carries no row_key (protobuf bytes default b""), so current_key was set to b"" instead of the actual row key.
• On commit_row, self._last_yielded_row_key = current_key wrote b"", corrupting the retry cursor. On any subsequent retryable error, _revise_request_rowset used b"" as last_seen_row_key, causing the already-committed row to be re-requested and yielded again.
• Also fixes the dead-code guard if current_key is None → if not current_key: protobuf bytes fields default to b"", not None, so the old check never fired.

Fix: move current_key = None to before the outer async for resp loop so it persists across response boundaries. The sync autogen file is regenerated from the async source.

Test plan

• Added test_chunk_stream_cross_response_row_key (async + autogen sync): constructs a two-response stream where the first response opens a row and the second commits it, then asserts _last_yielded_row_key equals the actual row key rather than b"".
• All 310 unit/acceptance tests pass (pytest tests/unit/data/_async/test__read_rows.py tests/unit/data/_sync_autogen/test__read_rows.py tests/unit/data/_async/test_read_rows_acceptance.py tests/unit/data/_sync_autogen/test_read_rows_acceptance.py).

[gemini-code-assist]

Code Review

This pull request fixes a bug in the chunk_stream method of the ReadRows operation where current_key was incorrectly reset to None at the start of each ReadRowsResponse. By moving the initialization of current_key outside of the response stream loop, the code now correctly handles rows that span across multiple responses when continuation chunks omit the row key. The changes are reflected in both the asynchronous implementation and the autogenerated synchronous counterpart, accompanied by new unit tests to verify the fix. I have no further feedback to provide as the changes are clean and well-tested.