settings: cross-field model validators should declare RELATED_FIELDS_CTX #558

Follow-up from #497 (PR #556).

When a cross-field validator fails without naming the fields involved (RELATED_FIELDS_CTX), lenient hydration can't tell which stored field to drop. It fails the whole module, which then falls back to defaults and loses every valid override. Dropping a guessed field instead could get around a security check, so the strict behaviour stays (test_lenient_model_level_error_still_raises).

Fix: go through every module's model_validator / cross-field field_validator and raise PydanticCustomError(..., {related_fields: [...]}), as background_tasks now does for its time limits. A doctor check could flag validators that don't declare it.

https://claude.ai/code/session_01M9neheZZEe3sVpDi2S3zT4