HBASE-30201 Skip system region moves during active transitions

[Sigma-Ma]

JIRA: https://issues.apache.org/jira/browse/HBASE-30201

What changes were proposed in this pull request?

Skip system-region moves when a transition is already scheduled or the source server is offline. Reuse balance() to validate the source location and handle submission failures per plan. Submit meta plans first and each plan only once.

Why are the changes needed?

During rolling upgrades, an existing region transition can cause the compatibility check to abort the remaining moves. Plans collected from earlier source servers can also be submitted repeatedly.

This change addresses both cases without changing the existing transition checks or crash-recovery procedures.

How was this patch tested?

mvn -pl hbase-server -am -Dtest=TestMoveSystemRegions,TestMoveSystemRegionsDuringRecovery,TestRaceBetweenSCPAndTRSP,TestSCPGetRegionsRace,TestAssignmentManager -Dsurefire.failIfNoSpecifiedTests=false -Dsurefire.rerunFailingTestsCount=0 test

mvn -B -pl hbase-server -am -Dsurefire.rerunFailingTestsCount=0 test