refactor(quickadapter): share Optuna-artifact quarantine path helper

refactor(quickadapter): share Optuna-artifact quarantine path helper

The best-params quarantine in Utils re-inlined the timestamp + tie-break
path computation that the journal quarantine already implemented in
QuickAdapterRegressorV3, duplicating the algorithm across the two
modules.

- Add a shared, parameterized _optuna_quarantine_path(path, now, *, tag,
  limit) in Utils (the module both call sites can import).
- Delegate from _quarantine_corrupt_optuna_best_params (Utils) and from
  _optuna_quarantine_journal (regressor); each keeps its own domain
  constants (journal vs best-params), logging and rename handling, so
  quarantine names, log output and error semantics are unchanged.
- Remove the regressor's private _optuna_quarantine_path copy.