Remove io.confluent:kafka-avro-serializer dependency from kafka-connect

[xinlian12]

Summary

Replace the Confluent KafkaAvroSerializer with a lightweight TestAvroSerializer that produces the same Confluent wire format (magic byte + schema ID + Avro binary) using the Schema Registry REST API directly.

Motivation

The io.confluent:kafka-avro-serializer dependency requires the Confluent Maven repository (packages.confluent.io), which is not Maven Central. This change eliminates that external repository requirement — all Maven dependencies now resolve from Maven Central.

Changes

• New: TestAvroSerializer.java — lightweight test-only serializer (~150 lines) that implements Serializer<GenericRecord> and talks to the Schema Registry REST API to register schemas and produce the Confluent wire format
• Modified: CosmosSinkConnectorITest.java — replaced KafkaAvroSerializer with TestAvroSerializer in 3 Avro integration tests
• Modified: pom.xml — removed Confluent Maven repository, kafka-avro-serializer dependency, and shade exclude for io.confluent:*
• Modified: external_dependencies.txt — removed cosmos_io.confluent:kafka-avro-serializer;7.6.0

Verification

• Build compiles successfully (mvn compile test-compile -DskipTests)
• No production code changes — all changes are test-scoped
• The Confluent Docker images (cp-kafka, cp-schema-registry, cp-kafka-connect) used by testcontainers are unchanged

[Copilot]

Pull request overview

Removes the io.confluent:kafka-avro-serializer dependency (and the Confluent Maven repository) from azure-cosmos-kafka-connect by introducing a small test-only TestAvroSerializer that produces the Confluent wire format directly against the Schema Registry REST API. This unblocks builds that cannot resolve from packages.confluent.io and limits dependencies to Maven Central.

Changes:

• Add TestAvroSerializer implementing Serializer<GenericRecord> with manual schema registration and Confluent magic-byte framing.
• Swap KafkaAvroSerializer for TestAvroSerializer in the three Avro integration tests in CosmosSinkConnectorITest.
• Drop the Confluent Maven repo, the kafka-avro-serializer dependency, the shade exclude for io.confluent:*, and its entry in external_dependencies.txt.

Show a summary per file

File	Description
sdk/cosmos/azure-cosmos-kafka-connect/src/test/java/com/azure/cosmos/kafka/connect/TestAvroSerializer.java	New lightweight test serializer that registers schemas via REST and emits Confluent wire format.
sdk/cosmos/azure-cosmos-kafka-connect/src/test/java/com/azure/cosmos/kafka/connect/CosmosSinkConnectorITest.java	Replaces KafkaAvroSerializer references with TestAvroSerializer in 3 Avro tests.
sdk/cosmos/azure-cosmos-kafka-connect/pom.xml	Removes Confluent repository, the Avro serializer dependency, and the related shade exclude.
eng/versioning/external_dependencies.txt	Removes the cosmos_io.confluent:kafka-avro-serializer version entry.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 0