fix(windows): put Git Bash coreutils on PATH for the non-login fallback

[OutThisLife]

What does this PR do?

Completes #63955. That PR made Hermes survive a broken Git Bash login shell (the classic Windows Directory \drivers\etc does not exist on bash -l) by falling back to a non-login bash -c. But a non-login shell never sources /etc/profile, so it never gets …\Git\usr\bin on PATH — and that directory holds every coreutil the file/terminal tools shell out to (cat, mktemp, mv, wc, head, stat, chmod, mkdir, find, …).

Net effect on an affected box: the fallback stopped Hermes from crashing, but then silently failed every write:

Tool write_file returned error: {"bytes_written": 0, "dirs_created": false,
  "error": "Failed to write file: ", "resolved_path": "C:\\Users\\...\\readme.txt"}

The error string is empty because the failure text went to a missing binary's stderr, and terminal commands returned exit 127 (command not found). So write_file → terminal echo/cat fallback both failed and nothing landed on disk.

Fix

Derive Git Bash's binary dirs (mingw64/bin, usr/bin, bin, …) from the already-resolved bash.exe and prepend them to the subprocess PATH on Windows, in /etc/profile precedence order so coreutils win over the same-named Windows System32 tools (find.exe, sort.exe) inside the shell.

• No-op off Windows (_git_bash_bin_dirs() returns []).
• No-op when the login snapshot is healthy — the snapshot re-exports the full login PATH inside the shell, so this only affects the broken-login fallback path.
• Fixes the whole class (every broken-login-bash Windows user), not one machine.

Adds _git_bash_bin_dirs() (derivation, cached) + _prepend_git_bash_dirs() (PATH merge), wired into _make_run_env.

How to test

scripts/run_tests.sh tests/tools/test_local_env_windows_msys.py — 40 passed. New TestGitBashCoreutilsOnPath covers:

• bin-dir derivation for PortableGit (<root>\bin\bash.exe) and MinGit (<root>\usr\bin\bash.exe) layouts
• exclusion of non-existent dirs; [] off Windows / when bash is unresolvable
• _make_run_env prepends coreutils before System32; idempotent; POSIX no-op

Checklist

• Conventional Commits
• Only changes related to this fix
• Added tests
• Cross-platform considered — Windows-only behavior, no-op elsewhere; validated on macOS host (derivation returns [], PATH unchanged)

[tonydwb]

Code Review Summary

Verdict: Comment

30 PRs reviewed (batch: 64044-63999). See aggregate summary at PR 64044.

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Reviewed by Hermes Agent