Reject UUID inputs for MIN and MAX

[alexeichhorn]

Problem

UUID is included in NullableAggregateOutput, so these calls compile:

dbkit::func::min(Event::id);
dbkit::func::max(Event::id);

PostgreSQL has no built-in MIN(UUID) or MAX(UUID) aggregate and rejects the query at execution time.

Expected behavior

• Remove UUID from the accepted MIN / MAX input set.
• Keep UUID support in models, comparisons, filters, and mutations unchanged.
• Add compile-fail coverage for required and nullable UUID columns.
• Avoid using one shared trait as both a nullability mapping and an inaccurate aggregate capability marker.

PostgreSQL extrema input types