Verify the pipeline compiles

Implement static bool GeneratedCatalogCompiles(string userSource): build a generator pipeline where a Gen2Attribute (emitted by the generator itself via RegisterPostInitializationOutput, AttributeTargets.Class) marks classes, and a generated static class GenCatalog2 exposes one method per marked class. Return true only when the OUTPUT compilation — user code AND generated code — has zero error diagnostics. The test proves the output check matters: a user class named GenCatalog2 collides with the generated type and must yield false.