Added support of writing union {string, bytes} to string

[chernser]

Summary

• Adds support of writing Avro message with field union {string, bytes} to String columns
  Note:
  unions should be resolved before passing records to the connector because
• using transformation is more straightforward way
• it keeps connector logic simple and therefor fast
• handling schema is extremely heavyweight task
  Currently we allow only this case because it is essential and simple to handle.

Closes #572

Checklist

Delete items not relevant to your PR:

• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided to include in CHANGELOG
• For significant changes, documentation in https://github.com/ClickHouse/clickhouse-docs was updated with further explanations or tutorials

[Copilot]

Pull Request Overview

This PR adds support for writing Avro messages with union {string, bytes} fields to ClickHouse String columns. The implementation handles union types by extracting the non-null value from the union and converting it appropriately for string storage.

Key changes:

• Enhanced schema validation to allow union {string, bytes} types for String columns
• Added union type handling in the data writing logic to extract and convert union values
• Extended test coverage with a new nullable union field

Reviewed Changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 3 comments.

File	Description
src/testFixtures/avro/test.idl	Added nullable union field description to test schema
src/testFixtures/java/com/clickhouse/kafka/connect/avro/test/Image.java	Generated Avro class updated with new description field and constructor
src/test/java/com/clickhouse/kafka/connect/sink/ClickHouseSinkTaskWithSchemaTest.java	Enhanced test to verify union field handling with database operations
src/main/java/com/clickhouse/kafka/connect/sink/db/ClickHouseWriter.java	Core logic for validating and writing union {string, bytes} to String columns

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

The error message is calling objSchema.schema().fields() but objSchema is already a Schema object, so this should be objSchema.fields() to avoid a potential NullPointerException or incorrect field access.

Suggested change

	objSchema.schema().fields(), colName));
	objSchema.fields(), colName));

[chernser]

even if it is primitive type it will have empty field.

[mzitnik]

LGTM