fix(mm): close TOCTOU race in _restore_incomplete_installs

[lstein]

Summary

This PR fixes an intermittent unit test failure in test_model_install.py.

• _restore_incomplete_installs snapshotted active sources under the lock, then iterated tmpdirs against the stale snapshot. A foreground import_model call landing during iteration could be missed, the same source enqueued twice, and the second download would fail with FileNotFoundError when trying to rename the .downloading file the first download had already moved.
• Fix: move the membership check inside the lock immediately before _install_jobs.append(job) so check-and-append is atomic. Duplicate-tmpdir suppression within a single restore pass stays outside the lock since it doesn't race against import_model.

Fixes #9141.

Test plan

• pytest tests/app/services/model_install/test_model_install.py::test_simple_download — ran 10× consecutively, all pass (was flaky ~30–50% on main)
• pytest tests/app/services/model_install/ — 31/31 pass

🤖 Generated with Claude Code

[JPPhoto]

This looks good when inspecting visually, but I'd also like to see a test added for this in tests/app/services/model_install/test_model_install.py, especially since it's a race condition. It may be a little challenging to get the test right, though:

1. Create a tmp install dir with an active marker for a remote source.
2. Monkeypatch _resume_remote_download() so restore pauses at a controlled point.
3. Insert an active job for the same source into _install_jobs while restore is between marker parsing and the new locked duplicate check.
4. Let restore continue.
5. Assert restore did not append a duplicate job and did not enqueue/resume a second download.

That maps directly to the PR’s fix around invokeai/app/services/model_install/model_install_default.py:249.

The only slightly fiddly part is choosing the pause point. Because the fixed code does the duplicate check under _lock immediately before append, the test probably needs to monkeypatch either _guess_source() or _read_install_marker() to signal “restore has observed the marker”, then add the foreground job before restore reaches the locked block. That is manageable with two threading.Events, similar to the existing test_import_waits_for_startup_restore.

The test should not need real network downloads if _resume_remote_download() is monkeypatched to fail the test if called for the skipped duplicate.