[magics] Fix `mypy` errors and re-enable it in CI

[dlqqq]

Description

Currently, mypy is only run for packages/jupyter-ai. The CI workflow is in .github/workflows/unit-tests.yml.

Moving forward into v3, we should enforce type checking throughout the entire codebase. This will reduce the number of bugs introduced by accident during development.

Proposed Solution

1. Run mypy packages/jupyter-ai-magics and fix all mypy errors (Claude Code can be extremely helpful here).

2. Add mypy packages/jupyter-ai-magics to the CI workflow.