fix(model-install): preserve install tmpdir when a single multi-file part fails

[lorenzozanee]

Summary

A single part error in a multi-file install deletes the entire install tmpdir. A transient failure on one file (an HTTP 5xx, or the sidecar rename race from #9432) discards parts that already completed and .downloading partials holding resumable progress, so the next attempt restarts the whole install from zero.

The error path now preserves the tmpdir when it contains anything worth keeping (a completed part file or a non-empty partial): the install is marked paused and the install marker is persisted, so restarting failed files and resume can pick up where the download left off. When nothing on disk is worth keeping, the tmpdir is still removed as before.

Related Issues / Discussions

Closes #9481

QA Instructions

New regression test (fails on the previous HEAD, passes with this change):

pytest tests/app/services/model_install/test_model_install.py::test_multifile_install_part_error_preserves_tmpdir

Surrounding suites and lint:

pytest tests/app/services/model_install/test_model_install.py tests/app/services/download/test_download_queue.py
uvx ruff@0.11.2 check invokeai/app/services/model_install/model_install_default.py tests/app/services/model_install/test_model_install.py

Result: 52 passed (35 model install, 17 download queue), ruff clean.

Merge Plan

Standard merge.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)