fix: handle channel removal gracefully in processCompletedReceives

[VimalN2005]

Fixes #2094

Problem

When a client disconnects abruptly, times out, or closes during request processing:

1. The channel is closed and removed from Selector.
2. Any completed receives buffered during the same poll loop still get processed in processCompletedReceives.
3. In SocketServer.scala, openOrClosingChannel(receive.source) returns None, which previously threw IllegalStateException(s"Channel ${receive.source} removed from selector before processing completed receive").
4. This exception was caught and logged as a loud, severe ERROR stack trace by processChannelException, flooding broker logs in production.
5. More importantly, receive.close() was never invoked on the orphaned receive, causing any allocated buffer in MemoryPool to leak until GC.

Solution

1. Graceful Handling in SocketServer.scala:
   • In processCompletedReceives(), when openOrClosingChannel returns None, log at debug level instead of throwing IllegalStateException.
   • Explicitly invoke CoreUtils.swallow(receive.close(), this, Level.WARN) to safely release receive.payload buffer back to MemoryPool.
   • Clean up any associated channelContexts via removeChannelContext(receive.source).
2. Unit Test in SocketServerTest.scala:
   • Added testCompletedReceiveWithRemovedChannel() verifying that an orphaned completed receive whose channel was removed from selector is processed cleanly without throwing an IllegalStateException or recording uncaught exceptions, keeping the processor healthy.

Verification

• Ran unit test locally:

  ./gradlew :core:test --tests "kafka.network.SocketServerTest.testCompletedReceiveWithRemovedChannel" -x checkstyleTest -x checkstyleMain -x spotbugsMain -x spotbugsTest

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