fix(stubgen): clean read-only staged files after failure - #4974

What

On Windows, a staged stub-generator file inherits a read-only destination's mode. If a later output cannot be staged or replacement is denied, cleanup cannot unlink that temporary file. It leaves the file behind and its PermissionError masks the original failure.

When temporary-file removal is denied, make that staged file writable and retry removal. The existing destination keeps its contents and read-only mode. Generated code, output permissions and the successful write path remain unchanged.

Tested

Windows, CPython 3.12.14, based on main at a373241de4403f7e8272be09ccf1a6b0866a8c69; tested commit 149aea506152466d0f0befa14c747105ab29f810:

• All 13 stub-generator tests pass, including two new regressions using actual Windows read-only files.
• Both new tests fail with the unmodified current upstream tools/stubgen.py and pass with the fix. One verifies the original missing-directory diagnostic and temporary cleanup; the other verifies cleanup after Windows denies replacing a read-only destination. Both preserve the original file and its mode.
• The full Python tools suite passes: 248 tests.
• Repository conventions and git diff --check pass.
• Reviewed the open stubgen-related work: fix(stubgen): write LF to stdout on Windows #3919 changes stdout newlines and test(tools): create hard links with os.link for Python 3.9 #4545 replaces a hard-link test API for Python 3.9. Neither changes this cleanup or implements the fix; their changes touch different lines.

No native runtime code changed. No full native build, CTest or title run was performed. Tests were run directly with Python; the Windows-specific replacement test is skipped on other platforms.

Checklist

• Based on current main; no other open PR implements the same functions
• One topic per PR; follow-ups go in a new PR
• No comments in code except technical debt
• Unimplemented paths throw (NotImplemented_nid_no_patch); silent stubs are listed under Silent stubs in their library's technical debt file; no native exports or silent stubs changed
• No notes, investigation .md files or images added to the repository
• New third-party code is a submodule built from source; none added
• Generic behaviour, not specific to one title
• Depends on: none
• Unblocks: none
• AI-assisted: yes (Codex)