Kafka Connect: validThroughTs() includes stale commitId entries from zombie coordinators

[thswlsqls]

Apache Iceberg version
main @ 49b89a8

Query engine
N/A — Kafka Connect sink, engine-agnostic

Please describe the bug
CommitState.validThroughTs() (kafka-connect/kafka-connect/src/main/java/org/apache/iceberg/connect/channel/CommitState.java line 146) computes the VALID_THROUGH_TS_SNAPSHOT_PROP snapshot property from readyBuffer without filtering by commitId. addReady() (line 61) buffers every DATA_COMPLETE event regardless of commitId, including ones from a zombie coordinator still emitting events for a previous commit. As a result, a zombie coordinator's stale (or null-timestamp) partition offsets can pull the valid-through timestamp into the past or suppress it entirely (null), even though the current commit's data is complete and valid.

PR #16453 ("Kafka Connect: Make CommitState.isCommitReady() O(1)", merged) added the same Objects.equals(currentCommitId, payload.commitId()) filter to receivedPartitionCount and covered it with testIsCommitReadyIgnoresZombieCoordinatorPayloads, but validThroughTs() was left unfiltered — an incomplete fix.

Steps to reproduce

1. Start a commit (startNewCommit()).
2. Have a zombie coordinator emit DATA_COMPLETE with a different commitId (addReady()).
3. Have the current coordinator emit its own DATA_COMPLETE.
4. Call validThroughTs(false) — the zombie's offsets are still included in the min-timestamp calculation.

Additional context
N/A — reproducible via a pure unit test on CommitState, no Docker required.