test: stabilize ChatGPT login subprocess on macOS

[mldangelo-oai]

Summary

• Flush the synthetic ChatGPT-login URL to stderr before explicitly terminating its child process.
• Prevent the ambient-credential authentication regression test from leaving a dangling Bun/Node subprocess or stalling macOS CI.
• Keep the change limited to the test fixture: no production authentication changes and no unrelated GitHub Actions upgrades.

Evidence

• The merged-main macOS job timed out in CodexSecurity orchestration > keeps ambient credentials available after successful ChatGPT login: https://github.com/openai/codex-security/actions/runs/30593828967/attempts/1
• Full SDK regression suite: 656 passed, 6 intentionally skipped, 0 failed, across 32 files.
• The exact previously flaky test passed all 12 concurrent stress attempts.
• The complete API authentication suite passed: 67 tests.
• Authentication process-boundary tests, TypeScript typechecking, Prettier, and git diff validation pass.

This intentionally does not claim to fix the broader production child-cancellation issue tracked separately in #131.

[mldangelo-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep it up!

Reviewed commit: 08fda92e21

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[chatgpt-codex-connector]

Security review completed. No security issues were found in this pull request.

Reviewed commit: 08fda92e21

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