Log per-op failures in ZkMultiWriteBuilder

[krishan1390]

Summary

When a multi-op ZK transaction fails, the rolled-up KeeperException does not tell the operator which op within the batch caused the failure. This adds per-op diagnostics so on a failed multi() we log, for each op in the batch, the path, the op type (create / set / delete / check), and the per-op result code from KeeperException.getResults().

• No behavior change — purely observability.
• Strengthens the existing testBadVersionAtomicRollback test to assert that KeeperException.getResults() is populated on failure (the precondition the new logging relies on).

Test plan

• ./mvnw -pl pinot-common -am test -Dtest=ZkMultiWriteBuilderTest
• Manual: trigger a BadVersionException against a live ZK and verify the new per-op log lines pinpoint the offender.

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