Fix CS8669 warning in generated MessagePack.GeneratedMessagePackResolver.g.cs - #2279

The generated MessagePack.GeneratedMessagePackResolver.g.cs emits CS8669 warnings when user types contain nullable reference type annotations in generic arguments (e.g., List<SomeClass?>), because the resolver template was missing the #pragma warning disable CS8669 already present in all formatter-generated files.

The resolver generates expressions like:

{ typeof(global::System.Collections.Generic.List<global::TempProject.SomeClass?>), 0 },

Without a nullable context or suppression, this triggers CS8669.

Changes

• ResolverTemplate.tt / ResolverTemplate.cs — Added #pragma warning disable CS8669 // We may leak nullable annotations into generated code. matching the pattern already used in FormatterTemplate.tt
• Resource files — Updated all 86+ MessagePack.GeneratedMessagePackResolver.g.cs test resource files to include the new pragma line
• New test — NullableReferenceTypeInListFormatter reproduces the exact scenario from the issue: a [MessagePackObject] with a required List<SomeClass?> Values property, verifying no diagnostics are emitted and the generated resolver contains the pragma