ci: add CI workflow to run tests and mypy on push/PR

[Bre77]

Intent

• Bootstrap CI: this repo has tests and a pyproject but no workflow runs them - the tag-triggered PyPI publish (python-publish.yml) ships with zero validation gate.
  • Adds .github/workflows/ci.yml running on push + pull_request.
  • Modeled on the fleet's reference CI (python-tesla-fleet-api, aiopowerwall): astral-sh/setup-uv, uv sync, matrix test job.
• Tooling choices matched to what this repo actually has, not introduced new:
  • Install: uv sync (repo already uses uv.lock).
  • Typecheck: mypy is already configured via [tool.mypy] in pyproject.toml but isn't declared as a dependency anywhere (no dev dependency group exists). Ran via uv run --with mypy mypy teslemetry_stream (ephemeral install) rather than adding a dev-dependency group, to avoid touching tooling config in a CI-bootstrap PR.
  • Lint: skipped - no ruff/flake8/etc. config exists in this repo, so none was introduced.
  • Tests: the existing tests/test_field_type_coercion.py is a plain script (if __name__ == "__main__"), not pytest-based - pytest would collect zero tests here since none of its functions are test_-prefixed. CI runs each tests/test_*.py directly via uv run python, matching the repo's actual test convention.
• Python matrix: 3.9-3.13, matching requires-python = ">=3.9" (no classifiers narrow this further).
• Explicitly out of scope (per one-issue-per-PR): python-publish.yml / the publish path is untouched. Wiring CI as a required gate before publish is a possible follow-up.

Testing

• Ran both CI commands locally before pushing: uv sync, uv run --with mypy mypy teslemetry_stream (passes, one pre-existing warning that mypy no longer supports the python_version = "3.9" target in [tool.mypy] - unrelated to this PR, not fixed here since it's a tooling-config change), and uv run python tests/test_field_type_coercion.py (all checks pass).
• Workflow itself validated by CI running green on this PR (see checks below).

Full narrative / original brief

E10.1 follow-up, from the fleet CI audit. Top finding for this repo: tests and pyproject exist but NO CI workflow runs them - the repo publishes to PyPI on tag with zero validation gate. Scope: add .github/workflows/ci.yml running on push + PR, matching the repo's tooling (uv), only add lint/typecheck if already configured (no new tool configs), Python matrix matching requires-python, modeled on python-tesla-fleet-api / aiopowerwall CI. Do not touch python-publish.yml in this PR.