ci: enable mypy and fail on mypy failures

[sneakers-the-rat]

currently there is a linting step to run mypy:

indeca/.github/workflows/ci.yml

Lines 92 to 96 in eb5068b

	- name: Run mypy
	id: mypy
	continue-on-error: true # Allow failure until codebase is fully typed
	run: |
	pdm run mypy .

however it fails (it doesn't even get to actually type checking because it's configured incorrectly) without marking the test run as failed because of the continue-on-error option: https://github.com/Aharoni-Lab/indeca/actions/runs/21266735522/job/61207662549?pr=52

mypy is great! and it would have prevented this bug because the type annotation doesn't match the default. it does take a bit of shifting time if the package isn't already strictly typed, so not trying to enable it now, but we should commit time to making the package pass mypy and then actually fail that action when mypy fails.