fix(cua-driver-rs)(macos): isolate and clean up window capture temp files

[z-x-yang]

Summary

• Generate a unique temporary path for every macOS window capture using the process ID, window ID, and a process-local atomic sequence.
• Own the path with an RAII guard so partial or unreadable capture files are removed on every return path.
• Add deterministic unit tests for per-call path uniqueness and cleanup-on-drop behavior.

Why

Window capture currently reuses /tmp/cua-driver-rs-capture-{window_id}.png.

Concurrent captures of the same window can therefore overwrite, read, or remove each other's files. Cleanup also happens only after a successful file read, so a failed screencapture invocation or read can leave a partial file behind.

A per-capture path removes the same-window collision, while RAII cleanup keeps failure paths from leaking temporary files.

Relation to #2104

This PR is intentionally separate from #2104.

#2104 adds display-asleep diagnostics and surfaces screenshot failures. This PR does not add or duplicate screenshot_error, text diagnostics, display-state detection, or action-result changes. It only hardens the temporary-file lifecycle used by window capture.

After #2104 is rebased onto current main, the changes remain complementary: its diagnostics run inside capture failure branches, while this PR ensures the associated temporary file is unique and removed when those branches return.

Test Plan

• rustfmt --edition 2021 --check crates/platform-macos/src/capture.rs
• cargo check -p platform-macos
• cargo test -p platform-macos — 134 passed, 0 failed
• git diff --check

The new tests deterministically verify unique paths and cleanup-on-drop without requiring Screen Recording permission or live TCC changes.

Caveats

• This focused change covers the window-capture temporary file extracted from the original fix; full-display capture handling is unchanged.
• A package-wide cargo fmt -p platform-macos -- --check currently reports pre-existing formatting drift in unrelated files, beginning with src/apps/mod.rs. The modified capture.rs passes a targeted rustfmt check, and unrelated formatting was intentionally left out of this PR.
• The current fix(cua-driver-rs)(macos): surface display-asleep state in capture errors and action results #2104 head predates capture error-handling changes already present on main and therefore requires rebasing independently of this PR.