fix: parse tracebacks after stderr prefixes

[alceops]

Summary

• parse Python tracebacks even when stderr contains prefix log lines before the traceback header
• keep the parsed traceback focused on the traceback block
• add a regression fixture/test for prefixed stderr output

Validation

• npm ci
• npm test -- --grep "parseError|prefixed logs" (4 passing)

Fixes #309.

[StantonMatt]

I fetched this branch locally to help with maintainer triage.

Validation:

• npm test -- --grep "prefixed logs" -> 1 passing
• npm test -> 44 passing

The fix matches the reported shape in #309: prefix stderr text is ignored for traceback parsing, while the parsed traceback still starts at Traceback (most recent call last):. I did not spot a conflict with the existing plain-traceback behavior in the full suite.

[PNHD]

Verified the current head against #309 on an LF-safe checkout. The prefixed-traceback regression, full npm test suite, repository Prettier check, and diff hygiene all pass locally. The change stays scoped to traceback parsing without disturbing the existing plain-traceback path.