Replace the baseline with a rename that a concurrent reader cannot block

[stroxler]

Summary:
test_baseline_prune_ignores_diagnostics_below_threshold fails intermittently on the Windows GitHub CI job (e.g. https://github.com/facebook/pyrefly/actions/runs/38726642921/job/116394023723). The server computes the prune, then logs while writing baseline to ...: failed to persist temporary file: Access is denied. (os error 5), and the test times out waiting for the rewrite.

write_baseline_file renames a temporary file over the baseline with NamedTempFile::persist, which on Windows calls MoveFileExW(MOVEFILE_REPLACE_EXISTING). That fails with ERROR_ACCESS_DENIED whenever another handle has the baseline open at that moment, such as the server's own read of the baseline while it computes diagnostics, or the test polling the file. Windows users can lose a prune the same way.

The baseline is now replaced with std::fs::rename. When MoveFileExW fails with ERROR_ACCESS_DENIED, the standard library retries the rename with FILE_RENAME_FLAG_POSIX_SEMANTICS, which replaces a file that others have open as long as their handles allow deletion; every handle the standard library opens does. Linux and macOS are unaffected, since a rename there never depends on open handles.

On Windows, NamedTempFile::new_in marks the file it creates as temporary and only persist clears the mark, so the temporary file is created with File::create_new through tempfile::Builder::make_in, which keeps the cleanup on failure. On Unix that file is no longer private to its owner, so a new baseline gets the usual mode for a new file (the umask) instead of a fixed 0o644. A rewrite still copies the permissions of the file it replaces.

Differential Revision: D125499135

[meta-codesync]

@stroxler has exported this pull request. If you are a Meta employee, you can view the originating Diff in D125499135.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[rchen152]

Review automatically exported from Phabricator review in Meta.