Fix provider cache atomic write temp path collisions

[juliusmarminge]

What Changed

• introduced a shared atomic file write helper that now uses Effect-native UUID generation and scoped temp directories
• switched provider cache persistence to use the shared helper
• added a regression test covering overlapping writes to the same provider cache file
• migrated the other duplicated server-side atomic JSON writers to the same helper for consistency

Why

Provider cache writes used temp filenames derived from process.pid and Date.now(). Two writes to the same cache file in the same process and same millisecond could pick the same temp path, causing one rename() to move the file and the other to fail with ENOENT. The helper now creates a scoped temp directory beside the target file and writes a UUID-named temp file before the final atomic rename, removing that collision class while keeping the implementation inside Effect APIs.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

  

Note

Fix provider cache atomic write temp path collisions using random UUIDs

• Adds makeAtomicWriteTempPath in atomicWrite.ts that generates unique temp paths using process.pid + crypto.randomUUID() instead of time-based suffixes, preventing collisions on concurrent writes.
• Adds writeFileStringAtomically as a shared Effect-based utility that handles directory creation, temp file write, rename, and cleanup.
• Refactors writeProviderStatusCache, persistServerRuntimeState, writeSettingsAtomically, and writeConfigAtomically to all delegate to this shared utility, removing duplicated atomic write logic.
• Adds a concurrent-write test for writeProviderStatusCache that verifies two parallel writes to the same file both succeed without data corruption.

^{Macroscope summarized fc43d5f. (Automatic summaries will resume when PR exits draft mode or review begins).}

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[juliusmarminge]

Usless test @cursor, remove

[juliusmarminge]

@cursor