Improve Kafka header mapping and ensure reliable Envelope propagation

[gardasar-code]

This change strengthens Kafka header handling by reliably mapping Kafka record headers to Wolverine Envelope headers and safely reading/writing individual header values. It ensures predictable conversion between Kafka byte[] headers and string values and guards against null header collections.

Why this is needed:

• Reliability: Prevents header loss.
• Predictable decoding: Provides a deterministic conversion from Kafka byte[] header values to strings, improving interoperability with external Kafka producers/consumers.
• Metadata propagation: Ensures application and system metadata (e.g., correlation, message type, and custom attributes) remain available on the Envelope for logging, tracing, and middleware.
• Transport correctness: Aligns behavior within the Kafka transport so that headers are consistently written and read across ingestion and publishing paths.
• Non-breaking: The changes are internal to the Kafka envelope mapping and do not alter public APIs or configuration surfaces.

Impact:

• More reliable header propagation across Kafka-based message flows.
• Reduced risk of runtime errors from missing header collections.
• Negligible performance impact and no API changes.

[jeremydmiller]

@gardasar-code Got it. This will be in either 4.10.1 or 4.11. Not sure yet about the next version number.