Add Message Translator enterprise integration pattern

[JerrettDavis]

PatternKit should add a first-class Message Translator enterprise integration pattern slice.

Sources:

• Enterprise Integration Patterns lists Message Translator in the message transformation catalog: https://www.enterpriseintegrationpatterns.com/patterns/messaging/MessageTransformationIntro.html
• EIP overview describes the broader integration pattern catalog: https://www.enterpriseintegrationpatterns.com/patterns/messaging

Scope:

• Fluent runtime path for translating Message to Message with header/context preservation policies.
• Source-generated path for attributed message translators.
• IServiceCollection integration for registering named translators.
• Real-world example around partner event normalization.
• Documentation, user guide, generator docs, and catalog entries.
• TinyBDD coverage for payload transformation, header preservation/filtering, failure handling, DI, and generated path.

Done when CI, docs, CodeQL, and coverage are green on main.