Ruff config missing mutable-default checks (B006, B008, B039, RUF012)

[enyst]

Context:
A recent review thread discussed avoiding mutable defaults for the Conversation visualizer parameter. Our Ruff configuration on main does not currently enable rules that catch mutable defaults and related pitfalls.

Proposed Ruff rules to enable:

• B006 (mutable-argument-default)
• B008 (function-call-in-default-argument)
• B039 (mutable-contextvar-default)
• RUF012 (mutable-class-default)

Current state:

• pyproject.toml only selects: E, F, I, UP, ARG
• pre-commit runs ruff check with project config
• CI runs pre-commit; no additional ruff rules enabled

Quick local run on main with these rules:

• 20× B008 (function call in default argument)
• 8× RUF012 (mutable class default)
• 2× B006 (mutable argument default)
• 0× B039 (mutable ContextVar default)

Impact:
Enabling these will enforce best practices around default values and prevent subtle shared-state bugs.

Plan:

1. Add these rules via extend-select in pyproject.toml
2. Land the config, then follow-up PRs to fix violations (or selectively allowlist safe patterns using extend-immutable-calls for B006/B008 where appropriate)

PR:
A PR enabling these rules has been prepared.