Fix subscription WAL retention at replay cursor

[Caideyipi]

Description

Problem

Subscription WAL retention only considered committed progress. Committed progress can advance on another consumer or replica while a local subscription queue is still replaying older WAL entries, allowing WAL deletion to pass the queue's unread replay cursor.

Changes

• Retain WAL from the earlier of the committed-progress boundary and the local replay cursor.
• Pass the generalized queue retention boundary through the subscription queue registry and retention calculator.
• Add a regression test covering committed progress ahead of the replay cursor.

Verification

• mvnw.cmd spotless:apply -pl iotdb-core/consensus,iotdb-core/datanode
• mvnw.cmd -nsu test -pl iotdb-core/datanode -am -Dtest=ConsensusPrefetchingQueueTest#testReplayCursorBoundsWalRetentionWhenCommittedProgressIsAhead -DfailIfNoTests=false -Dsurefire.failIfNoSpecifiedTests=false

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This PR has:

• been self-reviewed.
• added comments explaining the intent where it is not obvious.
• added a unit test covering the new code path.

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Key changed classes

• ConsensusPrefetchingQueue
• SubscriptionQueueRegistry
• SubscriptionWalRetentionCalculator
• IoTConsensusServerImpl