Fix race condition in FileStorage dequeue

[mcollina]

Summary

• Fix TOCTOU race condition when multiple workers try to claim the same job
• Use atomic rename() instead of read() + writeFileAtomic() + unlink()

Problem

The previous implementation had a race condition:

const message = await readFile(srcPath)      // 1. Worker A reads
// Worker B also reads here before A deletes
await writeFileAtomic(dstPath, message)      // 2. Both workers write
await unlink(srcPath)                        // 3. A deletes, B fails but already has data

Both workers end up with the same job in their processing queues.

Solution

await rename(srcPath, dstPath)  // Atomic - only one worker succeeds
return await readFile(dstPath)  // Only read after claiming

rename() is atomic on the same filesystem, so only one worker can successfully move the file.

Test plan

• All existing FileStorage tests pass
• Full test suite passes

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