[PR] KAFKA-20869: Guard stream thread addition for global-only topologies [kafka]

["lh0156 (via GitHub)" <gi...@apache.org>]

lh0156 opened a new pull request, #23022:
URL: https://github.com/apache/kafka/pull/23022

   ### Summary
   Prevent KafkaStreams#addStreamThread from creating a local StreamThread when the topology has no local processing tasks.
   
   ### Motivation
   Global-only topologies intentionally start with zero local StreamThreads, even when num.stream.threads is configured with a positive value. Calling addStreamThread after startup must preserve that topology invariant; otherwise it creates a StreamThread that cannot build local tasks and can crash the client or repeatedly trigger thread replacement.
   
   ### Testing
   - Added a regression test for a running global-only client that verifies no StreamThread is created and addStreamThread returns Optional.empty().
   - Verified the test fails before the fix because addStreamThread creates a StreamThread and reaches an internal NullPointerException.
   - ./gradlew :streams:check --no-build-cache --console=plain
   
   Fixes KAFKA-20869


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