Reuse DataRegion insert lock for IoTConsensus shutdown fencing

[Caideyipi]

Description

Follow up to #18841. IoTConsensus shutdown fencing currently acquires an additional region read/write lock around every apply. Reuse DataRegion.insertLock to check write admission and permanently close admission before storage and WAL shutdown. Memory backpressure and state-machine retry waits stay outside the insert lock.

Write successful insertions' WAL separators before releasing the insert lock, preserving their subscription metadata. Table deletions continue to release the lock during sealed-file I/O. Shutdown waits on a condition from the same lock until admitted deletions finish, including separator emission on success; failed deletions also unblock shutdown. Remove the state-machine apply wrapper.

Validation

• Targeted tests: IoTConsensusShutdownTest, DataRegionTest, DataRegionStateMachineTest, WALNodeTest, and testCompactionWithDeletionsDuringCompactions in both InnerSeqCompactionWithFastPerformerTest and InnerSeqCompactionWithReadChunkPerformerTest (74 tests, 0 failures/errors, 1 existing skip).
• Shutdown tests cover writers queued for the insert lock, admitted writes and pending WAL separators, memory backpressure, all storage write paths, unlocked table deletion I/O and failure, and other consensus protocols.
• Full reactor test compilation in English and Chinese: mvn test-compile -DskipTests and mvn test-compile -P with-zh-locale -DskipTests.
• mvn spotless:apply -pl iotdb-core/datanode.

This PR has:

• been self-reviewed for concurrent writes and shutdown.
• added or updated unit tests for the changed write and shutdown paths.
• added comments explaining the lock and deletion-draining behavior.