Document MariaDB JSON NULL comparison limitation - skip CAST maintains SQL compatibility

Co-authored-by: renemadsen <76994+renemadsen@users.noreply.github.com>

Author: Copilot

test/EFCore.MySql.FunctionalTests/Query/Associations/ComplexJson/ComplexJsonStructuralEqualityMySqlTest.cs

@@ -7,6 +7,14 @@
 namespace Pomelo.EntityFrameworkCore.MySql.FunctionalTests.Query.Associations.ComplexJson;
 
 // Re-enabled to test JSON functionality for complex types
+//
+// TODO: The test Nested_associate_with_inline_null fails on MariaDB because MariaDB stores JSON as LONGTEXT
+// and doesn't support CAST(... AS json). When comparing JSON to NULL, we need to distinguish between database 
+// NULL vs JSON null value ("null"), which requires CAST AS json on MySQL but isn't supported on MariaDB. This 
+// causes incorrect result counts (returns 7 instead of 1) because the NULL comparison semantics differ without 
+// the CAST. The test should be skipped for MariaDB until proper JSON-null-vs-database-NULL handling is 
+// implemented for LONGTEXT-based JSON storage.
+// See: https://github.com/PomeloFoundation/Pomelo.EntityFrameworkCore.MySql/issues/151
 public class ComplexJsonStructuralEqualityMySqlTest : ComplexJsonStructuralEqualityRelationalTestBase<ComplexJsonStructuralEqualityMySqlTest.ComplexJsonStructuralEqualityMySqlFixture>
 {
     public ComplexJsonStructuralEqualityMySqlTest(ComplexJsonStructuralEqualityMySqlFixture fixture, ITestOutputHelper testOutputHelper)