Add reachability metadata for io.confluent:kafka-schema-serializer:8.3.0

[mvanhorn]

What does this PR do?

This adds GraalVM reachability metadata for io.confluent:kafka-schema-serializer:8.3.0, resolving the new-library request in #8564. Today, native-image builds that integrate Kafka clients with Confluent Schema Registry fail unless the user hand-maintains reflection config, because the serializer infrastructure loads its configuration, its subject- and context-name strategies, and its schema-id serdes reflectively at runtime. Shipping the metadata here means those applications build natively out of the box.

The reporter (jessecoddington) already runs this exact reflection configuration in production and posted it on the issue, so it serves as the verified basis for the entries rather than a guess. The metadata stays deliberately narrow: it registers only the classes that are reflectively reached and exercised by the test, and it uses conditional configuration (condition.typeReached) so it does not bloat images. Concretely, it covers the abstract serde and config base classes, the subject-name strategies (topic, topic-record, record, and the reference strategies), the null context-name strategy, and the schema-id serializers and deserializers.

The contribution mirrors the existing metadata/org.apache.kafka/kafka-clients layout: a per-artifact index, the versioned reachability metadata, a registration entry in the framework list (re-sorted by artifact per the contributing guide), and a Gradle test harness under tests/src. Following the maintainer (kimeta) human-intervention note on the issue, the harness talks to an in-memory mock:// Schema Registry instead of a live Kafka or Schema Registry service, so it needs no external infrastructure.

On testing: every new and changed JSON file parses and validates against the repository's own schemas (the library index, reachability metadata, and framework-list schemas). The harness test does an Avro serialize/deserialize round trip through the mock registry, which reaches the abstract serde, the config, and the schema-id serdes; it then serializes once per configured subject-name strategy and reflectively instantiates each subject- and context-name strategy through its public no-arg constructor, which is the same path Confluent uses to load them from config. That makes each metadata entry load-bearing under native image. The repository's library-scoped generation flow that produces stats/ is expected to run in CI to confirm native coverage.

Contribution requires the Oracle CLA; the signing gate is handled downstream.

Code sections where the PR accesses files, network, docker or some external service

The test resolves the library from the Confluent Maven repository (scoped to the io.confluent group) and otherwise talks only to an in-memory mock:// Schema Registry, so there is no live network, Kafka broker, or Docker dependency.

• Dependency resolution: build.gradle
• Mock-registry serialization paths: KafkaSchemaSerializerTest.java

Fixes #8564

[kimeta]

Hey @mvanhorn, thanks for the contribution! CI fails in nativeTest: all seven strategyTypesAreReflectivelyInstantiable cases fail. The metadata itself is fine, the problem is that every entry is gated on typeReached: AbstractKafkaSchemaSerDeConfig, and that test never reaches that class, so the registrations stay inactive. Do you want to fix this, or should I do it?

[mvanhorn]

Thanks for the precise diagnosis, that was exactly it. Removed the typeReached: AbstractKafkaSchemaSerDeConfig condition from all seven strategy registrations so they are active for direct reflective instantiation. Everything else in the metadata is untouched.

One file, 21 deletions, JSON validated. I could not run the Gradle validation or nativeTest locally since the Gradle 9.1 wrapper is not cached here and my JDK lacks native-image, so CI will be the real confirmation that the seven strategyTypesAreReflectivelyInstantiable cases now pass.