HDDS-17125. OM transferLeadership is not idempotent under client failover retry - #12040

What changes were proposed in this pull request?

OM leadership transfer is not idempotent when a timed-out request is retried on the new leader. Although the requested target is already the leader, the handler starts another priority-setting / transfer / priority-reset cycle. This can overlap the original request and cause an unintended leader change.

This pull request:

• Returns success immediately when the requested target is already the leader, avoiding another priority update or transfer cycle while preserving existing client failover behavior.
• Adds regression coverage for repeated explicit-target requests after a successful transfer, verifying that the leader, term, configuration log index, and neutral priorities remain unchanged.
• Adds a negative test confirming that an unknown OM target fails without changing the leader.

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/HDDS-17125

How was this patch tested?

Local Test

mvn -pl :ozone-integration-test test \
  -Dtest=TestTransferLeadershipShell \
  -DskipShade -DskipRecon

GitHub Actions CI: https://github.com/echonesis/ozone/actions/runs/43381688173

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