[fix][ml] Fix active cursor position tracking after untracking - #26629

Motivation

When ActiveManagedCursorContainerImpl flushes an updateCursor(cursor, null) or add(cursor, null) operation, it removes the cursor from position tracking but retains its old position. Subsequent queries can return stale ranks, and restoring a position can leave the cursor detached or incorrectly linked. Repeated null updates can also attempt to unlink an already-untracked node and corrupt the remaining list.

This fixes a pre-existing container correctness issue identified while reviewing #26511. Normal broker deactivation uses removeCursor(), so these tests do not establish a failure in that production path.

Modifications

• Clear the stored position when incremental processing removes a node from position tracking.
• Clear the position and both list links when a rebuild excludes an untracked cursor, allowing subsequent incremental insertion.
• Only schedule removal for nodes with a tracked position, making null updates safe for already-untracked and never-tracked nodes.
• Add regression tests covering incremental and rebuild paths, repeated untracking, and restoration through both add() and real cursor seek callbacks.

Verifying this change

• Make sure that the change passes the CI checks.

The new tests start real ZooKeeper and BookKeeper services and create managed ledgers and cursors through ManagedLedgerFactoryImpl, without mocking cursors or containers. Null untracking is invoked through the container API; seek-based restoration exercises ManagedCursorImpl → ManagedLedgerImpl → the active cursor container.

All 14 new cases fail on the unpatched implementation with incorrect rank or slowest-position results. After the fix, all 73 cases across the three container test classes pass. quickCheck also passes locally.

./gradlew :managed-ledger:test \
  --tests ActiveManagedCursorContainerUntrackingTest \
  --tests ActiveManagedCursorContainerTest \
  --tests ActiveManagedCursorContainerRetentionTest \
  -PtestRetryCount=0 -PtestFailFast=false

./gradlew quickCheck

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• Dependencies (add or upgrade a dependency)
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