Fix MapStringEnumAsPostgresEnum re-probing entities via modelBuilder.Entity

[dmytro-khmara]

Summary

• MapStringEnumAsPostgresEnum<TEnum>() called modelBuilder.Entity(entityType.ClrType) on every entity to set the column type on its string-enum properties. That re-enters EF's entity-discovery conventions, which probe every CLR property on the type (and on any reference-typed nested types they pull in).
• On EF Core 10 models with a complex-collection element exposing e.g. Dictionary<K, V> configured via HasConversion(...), the probing fires before the user's fluent config has been applied. EF then throws Unable to determine the relationship represented by navigation '<element>.<property>' of type 'Dictionary<int, long>', breaking design-time DbContext construction (migrations, scaffolding).
• Fix: mutate IMutableProperty.SetColumnType(...) directly. Walks only existing model state; no convention re-entry. The per-property HasPostgresStringEnum<TEnum>() overload was already unaffected (it didn't go through the walker).

Repro

A new unit test, MapStringEnumAsPostgresEnum_DoesNotProbeCustomConvertedComplexCollectionProperties, configures a Tournament entity with a JSON-mapped Schedule complex property containing a ComplexCollection<Stage> whose Splits (Dictionary<int, long>) is round-tripped via a ValueConverter. Without the fix the test fails with the exact consumer error; with it, all 24 unit tests pass.

Test plan

• dotnet test ./test/StrEnum.Npgsql.EntityFrameworkCore.UnitTests — 24/24 pass on the fix branch
• Verified the new repro test fails on master (Unable to determine the relationship represented by navigation 'Stage.Splits' of type 'Dictionary<int, long>') and passes after the fix
• Smoke-test on the original consumer model that triggered the report