Allow STRING format in Kafka config

[charlespnh]

Please add a meaningful description for your change here

Fixes #35190

The docs says STRING is one of the valid format https://beam.apache.org/releases/yamldoc/current/#readfromkafka, but we're currently not validating for the STRING format here https://github.com/apache/beam/blob/master/sdks/java/io/kafka/src/main/java/org/apache/beam/sdk/io/kafka/KafkaReadSchemaTransformConfiguration.java#L50-L85...

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Mention the appropriate issue in your description (for example: addresses #123), if applicable. This will automatically add a link to the pull request in the issue. If you would like the issue to automatically close on merging the pull request, comment fixes #<ISSUE NUMBER> instead.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

To check the build health, please visit https://github.com/apache/beam/blob/master/.test-infra/BUILD_STATUS.md

GitHub Actions Tests Status (on master branch)

See CI.md for more information about GitHub Actions CI or the workflows README to see a list of phrases to trigger workflows.

[github-actions]

Checks are failing. Will not request review until checks are succeeding. If you'd like to override that behavior, comment assign set of reviewers

[chamikaramj]

Thanks @charlespnh. Were you able to validate that the source can indeed read in string format using an end-to-end test ?

cc: @derrickaw

[chamikaramj]

Retest this please

[chamikaramj]

Will merge when tests pass. Thanks!

[github-actions]

Assigning reviewers:

R: @ahmedabu98 for label java.
R: @fozzie15 for label kafka.

Note: If you would like to opt out of this review, comment assign to next reviewer.

Available commands:

• stop reviewer notifications - opt out of the automated review tooling
• remind me after tests pass - tag the comment author after tests pass
• waiting on author - shift the attention set back to the author (any comment or push by the author will return the attention set to the reviewers)

The PR bot will only process comments in the main thread (not review comments).

[charlespnh]

Thanks @charlespnh. Were you able to validate that the source can indeed read in string format using an end-to-end test ?

cc: @derrickaw

Thanks for the review! Discussed this offline already but for future reference, this PR change is verified in this Dataflow job in apache-beam-testing project https://console.cloud.google.com/dataflow/jobs/us-central1/2025-07-03_11_22_50-4738383819709098477;bottomTab=WORKER_LOGS;expandBottomPanel=true;graphView=0;logsSeverity=INFO?pageState=(%22dfTime%22:(%22s%22:%222025-07-03T18:22:51.215Z%22,%22e%22:%222025-07-03T18:24:04.238Z%22))&project=apache-beam-testing