fix: use atomic writes for FileCache to prevent race conditions

[Gosorasora]

Summary
Fixes race condition when multiple processes access the same cache file concurrently.

Related issue: wasmCloud/wasmCloud#4842

Problem
When two processes write to the same cache file simultaneously, one may read
an incomplete file, causing "unexpected end-of-file" errors.

Solution
Use atomic writes by writing to a temporary file first, then renaming it to
the final path. The rename operation is atomic on most filesystems.

Changes

• Modified put_data in FileCache to write to temp file first
• Added atomic rename after write completes