Give MappingQueryFbFixture its own test store.

MappingQueryFixtureBase uses the "Northwind" store name with a reduced model. FbTestStore creates a shared store
from the model of the first fixture that uses it, so whenever MappingQueryFbTest happened to run before the
Northwind tests, EFCore_Northwind.fdb was created without e.g. Products and Customers.City and every Northwind
test failed with "Column unknown"/"Table unknown".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: niekschoemaker

src/FirebirdSql.EntityFrameworkCore.Firebird.FunctionalTests/Query/MappingQueryFbTest.cs

@@ -57,6 +57,10 @@ public class MappingQueryFbFixture : MappingQueryFixtureBase
 	{
 		protected override ITestStoreFactory TestStoreFactory => FbTestStoreFactory.Instance;
 
+		// The base fixture shares the "Northwind" store, but FbTestStore creates a shared store from the model of
+		// whichever fixture uses it first. When this reduced model came first, all Northwind tests ran against its schema.
+		protected override string StoreName => "MappingQuery";
+
 		protected override string DatabaseSchema { get; } = null;
 
 		protected override void OnModelCreating(ModelBuilder modelBuilder, DbContext context)