fix(extensions): preserve subprocess errors for non-UTF8 stderr

[jbeckwith-oai]

This pull request fixes Codex subprocess error reporting when stderr contains non-UTF-8 bytes, as reported for the Windows npm shim in #5185.

Summary

Decode stderr with replacement on nonzero exit so callers receive RuntimeError with the exit code and readable diagnostic text instead of UnicodeDecodeError. Valid UTF-8 diagnostics remain intact; undecodable bytes become replacement characters.

The regression uses the captured CP950 diagnostic from #5185 through both Thread.run() and Thread.run_streamed(), with ASCII and UTF-8 controls. Scope is limited to failure stderr decoding.

Test plan

• Confirmed the two new CP950 cases fail against v0.22.3 with the reported UnicodeDecodeError.
• All 55 focused Codex execution/thread tests pass with the fix.
• Real-subprocess replay of captured CP950, ASCII, and UTF-8 stderr passes through both public methods on macOS. Native Windows shim execution was not independently reproduced.
• Full repository verification passed: formatting, lint, Mypy, Pyright, parallel tests, and serial tests. Native macOS sandbox tests are skipped locally under the prescribed Codex sandbox setting and run separately in CI.
• Independent review of the complete diff found no actionable issues.

Issue number

Closes #5185

Checks

• I've added new tests, if relevant
• I've run .agents/skills/code-change-verification/scripts/run.sh
• I've confirmed all verification steps pass
• If using Codex, I've run /review before submitting this PR

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-27T16:44:00.772330Z	450788d	PR opened
🔒 Security Review	✅ Completed 2026-09-27T16:44:56.507304Z	450788d	PR opened

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[markstuart-oai]

Reviewed at 450788d3d5c0d585a37b00d4b0a7a00d33066fb5. The replacement decode is in the existing nonzero-exit path after stderr is fully collected, so invalid bytes no longer mask the exit code while valid UTF-8 remains intact. The captured CP950 regression exercises both public run methods and checks the caller-visible error; the ASCII/UTF-8 controls preserve exact diagnostics. No blocking findings.

Verified all 22 hosted checks passed on this commit, including Windows tests. This was a source review; I did not run local tests or independently reproduce the native Windows npm-shim failure.