ci: enforce complete Python type checking

[afourniernv]

What

Makes CI type-check both shipped Python packages and treats mypy failures as blocking.

The workflow now uses the roots already configured in pyproject.toml instead of overriding them with only switchyard.

Why

The mypy configuration includes both switchyard and switchyard_rust, but CI currently passes switchyard explicitly and leaves the Rust binding facade unchecked. The job also uses continue-on-error, so type errors do not block a PR.

Both packages already pass under the existing strict configuration.

How tested

• uv sync --locked
• uv run mypy — no issues in 7 source files
• Added a temporary type error under switchyard_rust, confirmed mypy failed, removed it, and confirmed mypy passed again
• uv run ruff check .
• uv run pytest tests/ -v -m "not integration" — 117 passed, 2 deselected

No runtime behavior, public APIs, dependencies, packaging, or release behavior change.

[afourniernv]

For clarity, uv run mypy is not checking every Python file in the repo. It uses [tool.mypy].files, which currently limits the roots to switchyard and switchyard_rust. Mypy may read imported stubs and typed dependencies while checking those packages, but it will not sweep tests, scripts, benchmarks, or docs.