fix: stream errors to client before breaking generator loop

[Swigler]

Summary

• Stream errors are now delivered to clients — stream_result() is called with the error before breaking out of the generator loop
• Extra fields are preserved — fields like context and refresh_worker survive error extraction instead of being stripped

Fixes #397

What was wrong

Since v1.7.7 (PR #384), when a generator handler yields an error, the code in handle_job() breaks out of the streaming loop before calling stream_result():

if stream_output.get("error"):
    job_result = stream_output
    break                          # ← exits loop

await stream_result(...)           # ← never reached for errors

Clients listening on /stream receive normal chunks fine, but when an error occurs the stream goes silent. The error is saved internally (visible via /status), but never delivered through the stream.

Additionally, the error extraction replaced the entire dict:

stream_output = {"error": str(stream_output["output"]["error"])}

This discarded all other fields (context, refresh_worker, etc.).

PR #384's intent (marking errored jobs as FAILED instead of COMPLETED) is preserved — send_result() still receives job_result with the error and sets the correct status.

What changed

rp_job.py — handle_job() generator loop:

• Call await stream_result(session, stream_output, job) before break so errors reach the stream
• Preserve extra fields from the error dict using dict unpacking instead of replacing the whole dict

test_job.py — 2 new tests:

• Error is streamed before break (stream_result called with error payload)
• Nested error preserves extra fields (context, refresh_worker survive extraction)

Test plan

• All 31 tests in test_job.py pass (29 existing + 2 new)
• New test verifies stream_result is called twice (once for normal chunk, once for error)
• New test verifies extra fields (context, refresh_worker) are present in streamed error

[Swigler]

Ping — errors during streaming are swallowed because the generator breaks before sending. This streams the error to the client first. Ready for review.