fix: atomic credential file writes to prevent corruption on crash (fixes #42)

[jpoehnelt]

Fixes #42

Problem

All credential/token writes (credentials.enc, client_secret.json, token_cache.json) were non-atomic: a crash or Ctrl-C during the write could leave the file partially written or empty, permanently breaking auth.

Fix

• Added src/fs_util.rs with atomic_write(path, data) (sync) and atomic_write_async(path, data) (tokio) helpers that write to a sibling .tmp file then rename() into place — an atomic operation on POSIX filesystems
• Updated save_client_config in oauth_config.rs
• Updated save_encrypted in credential_store.rs (the primary credentials file)
• Updated save_to_disk in token_storage.rs (the token cache)

Tests

4 unit tests in fs_util.rs: creates file, overwrites existing, no leftover .tmp file, async variant

[gemini-code-assist]

Warning

You have reached your daily quota limit. Please wait up to 24 hours and I will start processing your requests again!

[changeset-bot]

🦋 Changeset detected

Latest commit: 0603bce

The changes in this PR will be included in the next version bump.

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR