KAFKA-20731: Streams: transitToUpdateStandby is called without checking ACTIVE_RESTORING, crashing the state updater thread

[arnabnandy7]

Fixes KAFKA-20731.

DefaultStateUpdater could call the non-idempotent ChangelogReader#transitToUpdateStandby method while the changelog reader was already in STANDBY_UPDATING. This could happen after all updating standby tasks were paused or removed because the empty-task state did not transition the reader back to ACTIVE_RESTORING. Adding or resuming a standby task afterward attempted the standby transition again, throwing an IllegalStateException and terminating the state updater thread.

This change centralizes changelog reader state transitions in DefaultStateUpdater and enforces the following invariants:

• When there are no updating tasks, the reader is in ACTIVE_RESTORING.
• When only standby tasks are updating, the reader transitions to STANDBY_UPDATING only if it is currently restoring active tasks.
• The existing non-idempotent contract and defensive exception in StoreChangelogReader#transitToUpdateStandby remain unchanged.

The transition helper is invoked after task additions, resumptions, pauses, removals, failures, and active restoration completion so that the reader state remains consistent with the updating task set.

The DefaultStateUpdaterTest changelog reader mock now models the actual reader state transitions. This allows the standby pause/resume regression test to fail if transitToUpdateStandby is invoked twice without an intervening transition back to active restoration.

Testing:

gradle streams:test \
  --tests org.apache.kafka.streams.processor.internals.DefaultStateUpdaterTest \
  --no-daemon \
  --console=plain

All 87 DefaultStateUpdaterTest tests passed. The test execution also completed checkstyleMain, checkstyleTest, and spotbugsMain successfully.