fix: changes to test that should help them pass on Windows under Bazel

[bolinfest]

#16460 was a large PR created by Codex to try to get the tests to pass under Bazel on Windows. Indeed, it successfully ran all of the tests under //codex-rs/core: with its changes to codex-rs/core/, though the full set of changes seems to be too broad.

This PR tries to port the key changes, which are:

• Under Bazel, the USERNAME environment variable is not guaranteed to be set on Windows, so for tests that need a non-empty env var as a convenient substitute for an env var containing an API key, just use PATH. Note that PATH is unlikely to contain characters that are not allowed in an HTTP header value.
• Specify "powershell.exe" instead of just "powershell" in case the PATHEXT env var gets lost in the shuffle.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Escape env value before using it as regex pattern

Using std::env::var(existing_env_var_with_non_empty_value) as the pattern for header_regex makes this matcher depend on the raw PATH contents, which often include regex metacharacters (especially backslashes on Windows like C:\Users\...). In that case the regex can either fail to compile or match incorrectly, so this test can still fail on the Windows/Bazel setup this change is targeting. Use header(...) for exact matching or regex::escape(...) around the value before passing it to header_regex.

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