`_save_anthropic_oauth_creds` wrote the Anthropic OAuth token file with
`os.replace(tmp, path)` followed by a post-hoc `chmod(0o600)`. Between the
rename and the chmod the token file existed at the default umask (0o644 on most
hosts) — a window in which another local user could read the access/refresh
tokens.
Write via `utils.atomic_json_write(..., mode=0o600)`, which creates the temp
with mode 0o600 *before* any content is written, fsyncs, atomically replaces,
preserves the existing file's owner, and cleans up its temp on failure. This
matches the `atomic_json_write(mode=0o600)` call already used elsewhere in this
module for the credential-pool write, and #56644's owner preservation.
Tests updated for the new mechanism, plus a check that the write goes through
`atomic_json_write(mode=0o600)` (mutation-verified).
b4289200ba