Add router context manager for asserting call counts

[nickita-khylkouski]

Summary

• add a Router.assert_call_count context manager for named routes
• count calls made only within the context while preserving existing call history
• expose the helper through the module-level API

Closes #248

Tests

• .venv/bin/pytest -q (330 passed, 2 skipped, 100% coverage)
• pre-commit hooks for all changed files