chore: update dev tooling and sync pre-commit with dependencies

[purificant]

The goal here is a general bump and sync for dev tooling to enable future contributions.

This PR updates the development toolchain and brings the pre-commit hooks, CI, and pyproject.toml dev dependencies onto the same versions. Contributors now get one consistent set of tools whether they run checks locally, in CI, or via poetry run, so local results match CI.

Changes

Upgrade ruff (0.9.10 to 0.15.17)
Brings in six minor releases of expanded lint coverage and faster checks. Lint and format pass clean across all files.

Upgrade mypy (1.20.1 to 2.1.0)
Adopts the latest type checker and aligns the dev dependency with the pre-commit hook. Type-checks pass clean over both postmark/ and tests/.

Upgrade Poetry to 2.4.1 everywhere
One consistent version across the pre-commit hook and both CI install steps, so local and CI environments match.

Add poetry-check and poetry-lock pre-commit hooks
Keeps pyproject.toml valid and poetry.lock in sync automatically, giving contributors fast local feedback on dependency changes.

Refresh pre-commit hooks
pre-commit-hooks v5 to v6, codespell to v2.4.2, bandit to 1.9.4, and adopt the current ruff-check hook id.

[gregsvo]

LGTM — dev tooling upgrade, no runtime changes.