Support external ScanForTypes handlers via nameof(Type.Method)

[Copilot]

ScanForTypes accepted Handler as a method name, but only resolved handlers on the containing type or on matched scanned types. This updates Handler = nameof(SomeType.Method) to also resolve static generic handlers declared on another referenced type/assembly.

• Handler resolution

  • Capture the declaring type from nameof(Type.Method) during attribute parsing
  • Resolve external static generic handlers during validation and source generation
  • Emit fully qualified invocations for external handlers

• Behavior preserved

  • Local and inherited handler methods continue to resolve as before
  • Static handler methods on matched scanned types still use the matched type path
  • Existing HandlerTemplate behavior is unchanged

• Docs

  • Clarify that Handler supports nameof(OtherType.Method)

• Coverage

  • Add focused tests for:
    • external handler methods returning collection items
    • external handlers with matched generic arguments
    • alias-qualified nameof expressions

Example:

[ScanForTypes(
    AssignableTo = typeof(ICommandHandler<>),
    Handler = nameof(Shared.Extensions.DecorateCommandHandler)
)]
private static partial IServiceCollection AddTransactionalProcessing(
    this IServiceCollection services
);

Generates calls like:

global::Shared.Extensions.DecorateCommandHandler<
    global::MyModule.CreateOrderHandler,
    global::MyModule.CreateOrder
>(services);