chore: CI uv migration, feature-flag checks, docs improvements

[pratyush618]

Summary

• CI: pip to uv -- lint and test jobs now use astral-sh/setup-uv@v5 + uv sync/uv run instead of raw pip/ruff/mypy/pytest
• CI: mypy on tests -- mypy now covers tests/python/ in addition to py_src/taskito/
• CI: feature-flag checks -- rust-test job now runs cargo check for postgres, redis, and native-async features
• CI: native-async in test builds -- Python test job builds with --features native-async so the 18 async tests actually exercise the feature
• Docs: MockResource -- expanded testing guide with return_value vs wraps, multiple resources, inject patterns
• Docs: local test commands -- added "Running Tests Locally" section

Test plan

• Linting passes locally (ruff check, mypy)
• CI workflow runs successfully on push