Context

PYTHON-5964 adopts the ruff 0.16 default rule set and suppresses the newly defaulted categories. That suppresses ~376 existing violations.

Violation counts and verification: ruff 0.16 upgrade plan gist.

PYTHON-6164 (PR #3126) dropped the stale py39 target-version. That change newly activates UP035, UP041, ASYNC109, UP042, UP017, and B905, which the PR ignores rather than fixes.

Definition of done

- Each category suppressed in PYTHON-5964 is either enabled with its violations fixed or kept ignored with a documented rationale in pyproject.toml.
- Each rule ignored in PYTHON-6164 (UP035, UP041, ASYNC109, UP042, UP017, B905) is either enabled and fixed or confirmed as a permanent ignore with a documented rationale.
- Lint and typing checks pass clean.

Pitfalls

- Some ignores are deliberate: ASYNC109 conflicts with CSOT timeout semantics, and UP042 changes _str_ behavior. Do not change runtime behavior to satisfy a rule.
- Enable categories one at a time to keep PRs reviewable.