[rebalance] Support preferred leader election goal

[sakshichitnis27]

Purpose

Linked issue: close #4086

Add a deterministic, manually triggered preferred leader election operation that restores bucket leadership to the first replica in each persisted assignment without moving replicas.

Brief change log

• Add the standalone PREFERRED_LEADER_ELECTION goal to the Java and Rust Admin APIs.
• Generate leader-only plans only when assignment[0] is alive, online, in the ISR, and not offline-tagged.
• Preserve replica assignments and reject combinations with other rebalance goals.
• Revalidate the target before execution and prevent fallback to another replica.
• Expose the goal through the existing Flink SQL rebalance procedure.
• Add focused unit and integration coverage for recovery, unavailable and offline-tagged replicas, assignment preservation, idempotency, and mixed-goal rejection.

Tests

Passed:

• ./mvnw -pl fluss-common -Dtest=GoalTypeTest test
• ./mvnw -q -pl fluss-server -am -Dtest=PreferredLeaderElectionGoalTest,GoalOptimizerTest,GoalOptimizerUtilsTest,LeaderReplicaDistributionGoalTest,ReplicaLeaderElectionTest -Dsurefire.failIfNoSpecifiedTests=false test
• ./mvnw -q -pl fluss-client -am -Dtest=RebalanceITCase#testPreferredLeaderElectionAfterRecovery -Dsurefire.failIfNoSpecifiedTests=false test
• ./mvnw -q -pl fluss-flink/fluss-flink-1.20 -am -Dtest=Flink120ProcedureITCase#testPreferredLeaderElectionProcedure -Dsurefire.failIfNoSpecifiedTests=false test
• ./mvnw -q -pl fluss-common,fluss-server,fluss-client,fluss-flink/fluss-flink-common -am -DskipTests validate

The full fluss-server suite ran 1,323 tests on the Oracle host. Seven unrelated storage tests failed because the host filesystem was above the tests' 85% disk-usage safety threshold and contained non-fresh test storage; no rebalance or leader-election tests failed.

Rust tests were not run locally because the Oracle host does not have cargo or rustc.

API and Format

Adds GoalType.PREFERRED_LEADER_ELECTION with wire value 3. No RPC schema or storage-format changes.

Documentation

Documents Java Admin and Flink SQL usage, standalone-goal behavior, and preferred-replica eligibility requirements.