DGS-21988 Fix transformation of nullable JSON props

[rayokota]

What

Checklist

• Contains customer facing changes? Including API/behavior changes
• Did you add sufficient unit test and/or integration test coverage for this PR?
  • If not, please explain why it is not required

References

JIRA:

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Open questions / Follow-ups

[Copilot]

Pull Request Overview

This PR fixes the transformation of nullable JSON properties by adding support for schemas with multiple types (e.g., ["string", "null"]). The fix ensures that when a field has multiple types, the correct subtype is selected based on validation before transformation is applied.

• Adds handling for multi-type schemas in the transform function
• Implements validateSubtypes function to select the appropriate type from multiple options
• Adds comprehensive test coverage for nullable field transformations with CEL field transforms

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
schemaregistry/serde/jsonschema/json_schema_util.go	Adds multi-type schema handling and validateSubtypes function
schemaregistry/serde/jsonschema/json_schema_test.go	Adds test schema and test case for nullable field transformations

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[sonarqube-confluent]

• 78.90% Coverage on New Code (is less than 80.00%)

Analysis Details

1 Issue

• 0 Bugs
• 0 Vulnerabilities
• 1 Code Smell

Coverage and Duplications

• 78.90% Coverage (55.70% Estimated after merge)
• No duplication information (0.00% Estimated after merge)

Project ID: confluent-kafka-go

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[Claimundefine]

LGTM