Follow the dict -> Pydantic BaseModel pattern established by #31514, #34080
and #34422: RetrievalService.external_retrieve now accepts
metadata_filtering_conditions: MetadataFilteringCondition | None directly
instead of dict[str, Any] | None.
The redundant internal MetadataFilteringCondition.model_validate(...) call
is removed; validation now happens at the call site in
HitTestingService.external_retrieve, following the "validate at the
boundary" principle cited in the issue.
Unit tests are updated to pass typed instances where appropriate and a
regression test is added for the None path. Behavior is otherwise
preserved. external_retrieval_model is left as dict[str, Any] because it
is an opaque payload forwarded to external retrieval providers.
Fixes part of #31497.
02fa84d853