Exclude polymorphic relations from extended search and fix missing JOIN on list queries

• SearchCollectionDecorator.get_fields now skips PolymorphicManyToOne and PolymorphicOneToOne relations during extended search, logging a debug message instead of traversing them.
• Query.add_join_relation replaces includes with left_joins so filtering/sorting on related fields uses a LEFT OUTER JOIN rather than eager loading.
• Query.query_aggregator now preserves both includes_values and left_outer_joins_values when composing queries without an initial where clause, preventing loss of join context.
• Behavioral Change: list queries that filter or sort on relation fields will now emit a JOIN instead of triggering eager loading, which changes query structure and may affect performance on large datasets.

^{Macroscope summarized 02d459a.}