Configure Confluent Schema Registry compatibility modes for JSON Schema and Protobuf subjects

Steps

1. Recognize that compatibility modes (BACKWARD, FORWARD, FULL, NONE, and their _TRANSITIVE variants) are the same mode set across Avro, JSON Schema, and Protobuf subjects, but the underlying evolution rules differ per schema type.
2. Set a subject-specific mode where needed instead of relying purely on the global default (BACKWARD), since Protobuf and JSON Schema evolution semantics don't always match Avro's.
3. Before registering a new schema version, test it with POST /compatibility/subjects/{subject}/versions/{version} (version accepts an integer or latest) and add ?verbose=true for failure reasons.
4. For Protobuf subjects, prefer BACKWARD_TRANSITIVE over plain BACKWARD/FORWARD — adding a new message type is not forward-compatible, so FORWARD mode rejects a change BACKWARD_TRANSITIVE would allow.
5. Re-run the compatibility check in CI against the actual target subject/version before a service deploy, rather than only checking once during local development.