HBASE-30433 Seed the master's flushedSequenceIdByRegion watermark with the region's final flushed seqid on region CLOSE - #8707

JIRA: https://issues.apache.org/jira/browse/HBASE-30433

Summary

Seed the master's flushedSequenceIdByRegion watermark with the region's final flushed seqid on region CLOSE, so a subsequent WAL split of a drained/crashed source RS recognizes already-durable edits instead of writing orphaned recovered.edits.

Why

The master already seeds this watermark with openSeqNum at region OPEN. That is the correct primary fence and fires on every reopen path, but it leaves one narrow graceful-close window uncovered:

1. A region is gracefully closed on the source RS (memstore flushed to the close marker).
2. The source RS dies before the target RS finishes OPEN, so the OPEN-time seed has not fired.
3. The source RS's SCP splits its WAL in that window, filtering against a still-stale flushedSequenceIdByRegion, and writes a (harmless-but-present) recovered.edits file.

Seeding on CLOSE advances the watermark to the region's reported flushed seqid before the source RS dies, closing this sub-window for the graceful-close case. This is additive: it does nothing for the pure-crash / never-gracefully-closed path (there is no close report), so the OPEN-time seed remains. Both writers go through the same monotonic merge(Math::max), so they compose safely and never regress a higher value.

Change

No protobuf/RPC change — the RegionStateTransition message already carries an optional openSeqNum field and the CLOSED transition is already routed through the same master-side handler.

• RegionServer — CloseRegionHandler and UnassignRegionHandler report HRegion.getMaxFlushedSeqId() on the CLOSED transition (was HConstants.NO_SEQNUM). HRegionServer.createReportRegionStateTransitionRequest now sets the wire openSeqNum field for CLOSED too (when >= 0), not just OPENED.
• Master — AssignmentManager seeds the watermark on the CLOSED transition via serverManager.reportRegionOpen(regionInfo, seqId) when seqId >= 0 (the existing atomic max-merge).

Test

Adds TestGetLastFlushedSequenceId#testFlushedSequenceIdSeededOnRegionClose: write + flush past openSeqNum, disableTable (a graceful close that keeps the region — unlike delete/split/merge, disable does not call ServerManager#removeRegion), then assert the watermark reflects the post-close flushed seqid.

Local run on current master (JDK17): TestGetLastFlushedSequenceId 3/3, TestServerManager 5/5, TestMergeTableRegionsProcedure 9/9, TestAssignmentManagerUtil 3/3, TestMaster 7/7, TestDLSFSHLog 3/3, TestDLSAsyncFSWAL 3/3 green; spotless:check on hbase-server clean.

Note on stacking

Builds on #8584 (HBASE-30335, now merged to master), which introduced the ServerManager.reportRegionOpen seed this reuses. Rebased onto master; this PR is now the single CLOSE-time commit.