[trejjam]

fix: invoke static interface mapping methods on the interface type

Description

When a mapper (or an external mapper used via [UseMapper] / [UseStaticMapper]) implements an interface that declares a static method, Mapperly picks the method up as a user-implemented mapping. It then calls it through a cast instance, which does not compile:

public interface IMapperBase { public static string MapValue(string value) => value.ToUpper(); }
[Mapper] public partial class Mapper : IMapperBase { public partial B Map(A source); }

// before
target.Value = ((global::IMapperBase)this).MapValue(source.Value); // CS0176
// after
target.Value = global::IMapperBase.MapValue(source.Value);

The same cast was emitted for existing-target (void) mappings, non-inlinable queryable projections, static partial mapping methods (CS0026, this in a static method), [UseMapper] members (CS0176) and [UseStaticMapper] types (CS0119, ((global::IOther)global::Other)).

Four commits:

1. fix: invoke static interface mapping methods on the interface type: UserImplementedMethodMapping and UserImplementedExistingTargetMethodMapping call static interface methods on the fully qualified interface type instead of casting the receiver. Output for all other mappings is unchanged.
2. fix: ignore static abstract and static virtual interface mapping methods: these members can only be accessed through a type parameter (CS8926), so global::IBaseMapper<global::Mapper>.BaseMapping(...) does not compile either. UserMethodMappingExtractor now treats them as invalid user mappings: ignored, or RMG001 when marked with [UserMapping] (unless Ignore = true). An implicit implementation in the mapper (public static D BaseMapping(C source)) is still discovered and used as before.
3. fix: honor ignored user mappings before reporting unsupported signatures: a method marked with [UserMapping(Ignore = true)] is skipped before its signature is validated, so an ignored static abstract member (or any other ignored method with an unsupported signature) no longer reports RMG001.
4. docs: document user-implemented mapping methods in base classes and interfaces: adds a short section to the user-implemented methods docs.

The change is covered by unit tests that assert the generated code. No integration tests were added.

Checklist

• I did not use AI tools to generate this PR, or I have manually verified that the code is correct, optimal, and follows the project guidelines and architecture
• I understand that low-quality, AI-generated PRs will be closed immediately without further explanation
• The existing code style is followed
• The commit message follows our guidelines
• Performed a self-review of my code
• Hard-to-understand areas of my code are commented
• The documentation is updated (as applicable)
• Unit tests are added/updated
• Integration tests are added/updated (as applicable, especially if feature/bug depends on roslyn or framework version in use)

[trejjam]

This is a niche issue. I didn't find it in my code. An agent did while I was working on the #2240 . I did not find it super important; if you do not want to spend time on it, just close it.

I reviewed the code, and it seems correct.