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 temp path collisions in provider cache atomic writes by extracting writeFileStringAtomically

• Introduces atomicWrite.ts with a writeFileStringAtomically utility that writes to a UUID-named temp file in a scoped temp directory co-located with the target, then renames it to the final path.
• Replaces duplicated inline atomic-write logic (mkdir, write temp, rename, ensure cleanup) across providerStatusCache.ts, serverRuntimeState.ts, serverSettings.ts, and keybindings.ts with calls to the new utility.
• The original implementations shared a fixed temp filename suffix, causing collisions under concurrent writes; the UUID-based naming in the new utility prevents this.

^{Macroscope summarized 6ad9e56.}

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[macroscopeapp]

Approvability

Verdict: Approved

This PR extracts duplicate atomic file write logic into a shared utility and improves temp file path generation to use UUIDs instead of pid+timestamp, addressing potential collision issues. The changes are mechanical consolidation with a minor reliability improvement, self-contained with no API or schema changes.

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