Fix bulk insert for EF Core JSON complex collections (ComplexCollection.ToJson)

[Copilot]

Bulk inserting entities with ComplexCollection(...).ToJson() mappings failed with ArgumentException: Property 'X' is not defined for type 'List<T>' because TableMetadata.GetColumns tried to expand the element-type sub-properties onto the collection type.

Core fix

TableMetadata.GetColumns now detects JSON-mapped complex properties via the Relational:ContainerColumnName EF Core annotation and emits a single ColumnMetadata for the whole JSON column instead of expanding sub-properties.

ColumnMetadata gains a second constructor for JSON complex properties that serializes the collection with System.Text.Json default options (PascalCase — matching EF Core's storage format). Property is now IProperty?.

// Now works — previously threw ArgumentException during metadata construction
builder.ComplexCollection(x => x.DemandLegalEntityDistributions, x => x.ToJson());
await dbSet.ExecuteBulkInsertAsync(entities);

Provider fixes

• PostgreSQL — GetPostgreSqlType guards column.Property != null; falls back to parsing StoreDefinition to resolve NpgsqlDbType.Jsonb / NpgsqlDbType.Json.
• SQLite — GetSqliteType guards column.Property != null; falls back to StoreDefinition prefix for the store type.

Tests

Added TestEntityWithJsonComplexCollection and an InsertsEntities_WithJsonComplexCollection test covering both null and non-null JSON collections. InsertReturn/InsertReturnAsync strategies are skipped on SQLite — EF Core requires composing JSON path expressions over the result, which is incompatible with a raw INSERT...RETURNING statement.