[core] Cover snapshot sequence bucket rescale check

[Aitozi]

Purpose

This PR makes write-only snapshot sequence APPEND commits participate in the same bucket-number consistency check as unordered write-only append commits. It prevents INSERT INTO from writing new files with a rescaled bucket count before existing data layout is rewritten, while INSERT OVERWRITE can still be used to rescale the layout.

Tests

• git diff --check HEAD^ HEAD
• mvn -s ~/.m2/apache-community.xml -pl paimon-core -am -Pfast-build -DfailIfNoTests=false -Dtest=FileStoreCommitTest#testWriteOnlySnapshotSequenceCommitChecksRescaledBucketNumber test

[JingsongLi]

Reviewed the change and the added coverage. The widened same-bucket check is scoped to APPEND commits on fixed hash buckets and now also covers write-only snapshot sequence initialization, which is the path that can otherwise skip restoring previous file metadata during writes. OVERWRITE still bypasses the check, so the supported bucket-rescale flow remains available.

I also ran the focused core regression test locally:

mvn -pl paimon-core -Pfast-build -Dtest=FileStoreCommitTest#testWriteOnlySnapshotSequenceCommitChecksRescaledBucketNumber test

LGTM from my side.