Add Message Filter enterprise integration pattern

[JerrettDavis]

Add a first-class Message Filter pattern to PatternKit.\n\nAcceptance criteria:\n- Fluent runtime API for accept/drop decisions over messages\n- Source-generated path for typed filter factories\n- Production-style example importable through IServiceCollection and AddPatternKitExamples\n- Docs for runtime, generator, and example usage\n- TinyBDD runtime, generator, example, and catalog coverage\n- Pattern coverage catalog entry with no generator gap