🔒 Fix insecure temporary file creation in atomic_write

[Cylae]

🎯 What: Replaced the predictable temporary filename generator (format!(".tmp.{}.{}.{}", file_name, pid, nanos)) in atomic_write / atomic_write_str with tempfile::Builder to create unguessable, cryptographically secure temporary files using OS primitives (mkstemp/O_TMPFILE).

⚠️ Risk: The previous filename generation scheme relied on predictable parameters (process ID and system time nanoseconds), exposing temporary file creation to race conditions, symlink attacks, or file squatted collisions when writing to directories with shared access.

🛡️ Solution: Added tempfile = "3.8" to Cargo.toml and updated atomic_write in server_manager/src/core/atomic_io.rs to use tempfile::Builder::new().prefix(&prefix).tempfile_in(parent). Added POSIX permission controls on creation, flushed and synced buffers, and atomically persisted the temporary file to the destination path using temp_file.persist(dest).

---

PR created automatically by Jules for task 10188298294900936929 started by @Cylae

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.