KAFKA-12986: Validate throttled replica entries before parsing

[AkshathaTR13]

Summary

• Reject leading and internal empty entries in throttled replica lists while preserving reset values and trailing commas.
• Reject partition or broker IDs that cannot be parsed as Java integers.
• Trim individual entries consistently in the runtime parser.
• Add regression coverage for the Jira reproducer and related parser/validator mismatches.

Testing

• git diff --check
• Direct 22-case validation/parsing matrix: all cases passed.
• The targeted Gradle test could not start in the local workspace because Gradle 9.7.1 could not be downloaded; GitHub CI will run the repository tests.

Generated-by: OpenAI Codex

[AkshathaTR13]

Hi Kafka maintainers, this is my first Apache Kafka code contribution. When convenient, could someone please approve the CI workflow and review this fix? Thank you!