fix: use SOURCE_ERROR in streaming runner commit-failure path

[anaselmhamdi]

Summary

• PipelineReturnStatus.ERROR does not exist on the enum — when source.commit() raised in StreamingRunner.run(), the except branch crashed with a secondary AttributeError: ERROR instead of reporting failure cleanly.
• Swap both references in bizon/engine/runner/adapters/streaming.py:168-169 to PipelineReturnStatus.SOURCE_ERROR, matching the convention already used in bizon/engine/pipeline/producer.py.

Observed in production

Datadog traceback:

File ".../bizon/engine/runner/adapters/streaming.py", line 168, in run
  monitor.track_pipeline_status(PipelineReturnStatus.ERROR)
File ".../python3.11/enum.py", line 786, in __getattr__
  raise AttributeError(name) from None
AttributeError: ERROR

Downstream impact (verified safe)

• track_pipeline_status(...): value is interpolated into a statsd tag. SOURCE_ERROR is already emitted elsewhere (producer.py), so no new metric cardinality.
• RunnerStatus(stream=SOURCE_ERROR): validates against Optional[PipelineReturnStatus]; is_success returns False; to_string() renders "Pipeline finished with status Failure (Stream: source_error)".
• bizon/cli/main.py:136 branches on result.is_success → failure path runs as intended (previously it never got there because of the crash).

Test plan

• uv run ruff check bizon/engine/runner/adapters/streaming.py — clean
• grep -rn "PipelineReturnStatus.ERROR\b" bizon/ returns no matches
• Smoke test: RunnerStatus(stream=PipelineReturnStatus.SOURCE_ERROR) → is_success=False, to_string() renders correctly
• Merge and observe that streaming-runner commit failures now log source_error cleanly instead of AttributeError: ERROR

Version bump will follow in a separate PR.

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