[GSoC 2026] Kafka Streams runner: ask for primitive reads in the Java wrapper

[junaiddshaukat]

Part of #18479. Follows up #39752 (comment).

A Read expands into a splittable DoFn by default, which this runner does not translate, so a pipeline that merely reads would fail to translate unless it knew to call SplittableParDo.convertReadBasedSplittableDoFnsToPrimitiveReads itself. The measurement application does exactly that, which is what prompted the question.

KafkaStreamsRunner now sets use_deprecated_read and converts the pipeline before handing it to the portable runner, so the proto that reaches the job server already holds primitive reads.

Worth being precise about what each half does. Beam already converts unless a pipeline asked for splittable reads, so the change that matters is that the runner calls the conversion at all; the experiment covers the case where a pipeline asks for use_sdf_read, which this runner cannot honour. There is a test for each, and each fails if its half is removed.

This is the Java wrapper only. A Python pipeline builds its proto client-side and does not pass through here.

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