Harden mypy/ruff tooling and improve typing

[Carreau]

Toolchain:

• Bump pinned linters to what CI actually enforces: ruff 0.2.0 -> 0.15.8
  and mypy 1.8.0 -> 1.18.2. The old mypy pin masked errors that the
  already-declared test-dependency range (>=1.17,<1.19) reports; the code
  now passes cleanly under both 1.18.2 and 1.19.
• Enable additional mypy error codes: truthy-iterable, possibly-undefined,
  redundant-self, unused-awaitable.
• Drop ruff rules removed upstream (UP038, PT004, PT005); ignore PLC0415
  (traitlets relies on lazy/optional imports); relax RUF059/PLW0108/B024
  for tests, matching the existing per-file test allowances.

Typing / correctness:

• parse_notifier_name / observe / unobserve: Iterable -> Collection, so the
  not names / All in names checks are honest (a generator was a latent
  bug) and truthy-iterable is satisfied.
• SingletonConfigurable.instance now returns Self instead of a bespoke
  TypeVar (PYI019).
• _CallbackWrapper gets a hash consistent with eq (PLW1641).
• Use is instead of == for exact trait-type comparisons (E721).
• Correct/remove stale type: ignore codes and unused noqa directives.

Cleanups:

• Modernize %-formatting and str.format to f-strings (UP031/UP032).
• Replace dict-index iteration with .items() (PLC0206), iterable unpacking
  (RUF005), and lazy %-args for a logging call (G004).
• Reformat with the newer ruff-format.

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01TBmDBHmq8vVSGig4i37KK7