Fix CI workflow evaluation and Python matrix

[mitmul]

Summary

This updates the GitHub Actions workflow so CI jobs are created correctly again and the MLX integration lane runs on a supported Python version.

Root cause

Recent workflow runs were failing before any job started because the workflow used ${{ runner.temp }} in job-level env, where the runner context is not available during workflow evaluation. The most recent failed run showed Invalid workflow file annotations and produced no jobs.

A separate earlier failure also showed that mlx does not currently publish macOS wheels for Python 3.14, so the integration job could not complete uv sync on that interpreter.

Changes

• add pull_request as a trigger so CI also runs for PRs
• move cache directories from ${{ runner.temp }} to repository-relative paths
• run the mock test matrix on ubuntu-latest
• keep the MLX integration test on macos-latest but pin it to Python 3.13
• update the integration job's uv cache key to match Python 3.13

Validation

• uv run --python 3.13 pytest -s tests/test_cli.py tests/test_warning_filters.py
• uv run --python 3.13 python - <<'PY' import mlx import mlx_lm print('mlx ok') print('mlx_lm ok') PY