feat(inkless): POD-2457 Allow transactional offset commits for Diskless sources

[AnatolyPopov]

Permit TxnOffsetCommitRequest to reference Diskless topic-partitions as offset keys, enabling transactional offset commits for consumers reading from Diskless topics.

[Copilot]

Pull request overview

This PR enables transactional offset commits for consumers reading from Diskless topics by allowing TxnOffsetCommitRequest to use Diskless topic-partitions as offset keys, while keeping transactional production to Diskless topics unsupported.

Changes:

• Removes the Kafka API rejection of Diskless topics in transactional offset commit requests.
• Updates unit and integration tests to cover successful transactional offset commits for Diskless sources.
• Updates Inkless feature documentation to distinguish transactional offset commits from unsupported transactional production paths.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
core/src/main/scala/kafka/server/KafkaApis.scala	Allows validated Diskless topic partitions to flow to the group coordinator for transactional offset commits.
core/src/test/scala/unit/kafka/server/KafkaApisTest.scala	Updates the Diskless transactional offset commit unit test to expect success and verify coordinator delegation.
core/src/test/java/kafka/server/InklessLegacyClusterTest.java	Adds an end-to-end legacy cluster test for sendOffsetsToTransaction with a Diskless source topic.
docs/inkless/FEATURES.md	Documents transactional offset commits as supported and clarifies remaining unsupported transactional operations.

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[viktorsomogyi]

Do you have some broader context here? Why can we allow transactional offset commits?