test: add regression test for AutoMQ target-node partition change builder

[sivaramanr38]

This PR adds a regression test for the AutoMQ‑specific target‑node leader election path in the partition change builder.

What Changed
Introduced a new test in ElasticPartitionChangeBuilderTest.java that exercises the setTargetNode behavior through the change‑record build path.

The test verifies that the emitted change record:

• assigns the leader,
• sets the ISR to the target node,
• sets the replica list to the target node,
• marks leader recovery as recovered.

Why This Is Useful

• Strengthens coverage for AutoMQ‑specific partition change logic.
• Provides confidence in a critical leader election path.
• Helps prevent regressions without altering production code.

Testing Strategy

• Verified correctness through the new unit test.
• CI pipeline validation will ensure full test execution and integration.

[CLAassistant]

All committers have signed the CLA.