CI: test the supported Python range, and run ruff + mypy #42

CI currently runs a single job — pytest on Python 3.12 only. Two gaps worth closing.

1. Test the supported Python range

pyproject.toml declares requires-python = ">=3.10", but .github/workflows/ci.yml pins one version with no matrix:

- name: Set up Python
  run: uv python install 3.12

So neither end of the supported range is exercised: 3.10 and 3.11 are untested, and local development is currently on 3.14.6 (what uv resolves by default), which CI never sees either. Worth a matrix over 3.10–3.12 at minimum, and probably 3.13/3.14 too since that is what the code is actually being written against.

Low but non-zero risk today — the package uses from __future__ import annotations throughout and X | None syntax, so 3.10 compatibility is plausible but unverified.

2. Run ruff and mypy in CI

Both are dev dependencies and both are configured ([tool.ruff.lint] with a decent rule selection, [tool.mypy] with strict = true), but neither runs in CI. Lint and type regressions can land on main freely.

Blocker to be aware of for mypy: there are currently 18 pre-existing errors, all no-untyped-def (missing return annotations) in test files:

tests/test_update_changelog.py
tests/test_tools.py
tests/test_package.py
tests/conftest.py

Package source (hotdata_langchain/) is clean. So adding mypy to CI needs those test annotations added first, or a baseline — it cannot just be switched on. Ruff passes clean today and can be added immediately.

Context

Worth doing, not urgent — filed after a deliberate decision to prioritise capability work while the package has no external consumers. The gap became visible while validating the hotdata-framework 0.9.0 bump (#35): its green check covered 24 tests on one Python version and none of them touched upload_parquet, the one function whose internals actually changed. The 0.4.1 breakage it fixed was found by running against the live engine, not by CI — so this is about catching regressions cheaply, not about replacing live verification.