ci: coverage, python matrix, uv caching

[michaeln31]

Summary

Phase 0 / F3: extend the test job in ci.yml with coverage reporting, a Python version matrix, and uv-based cached installs.

• pytest-cov (report-only): adds --cov=custom_components/andersen_ev --cov-report=term-missing --cov-report=xml to the pytest invocation. No --cov-fail-under threshold — the 95% hard gate is deliberately deferred to the Silver phase (S6/Silver Gate).
• Python matrix ["3.13", "3.14"]: PyPI confirms homeassistant==2026.1.3 has requires_python >= 3.13.2 and declares classifiers for Python 3.13 and 3.14 only. Both versions are included; no guessing beyond what the package metadata states.
• uv caching: replaces actions/setup-python + pip install with astral-sh/setup-uv@v5 (enable-cache: true, python-version from matrix) and uv pip install --system. Keeps install steps simple and idiomatic.

lint, hassfest, and hacs jobs are unchanged.

Test plan

• CI run is green for both matrix legs (Python 3.13 and 3.14)
• Coverage report appears in the test job logs (terminal output + xml artifact)
• No coverage threshold failure (report-only)
• uv cache is populated on first run, hit on subsequent runs