[flink] Support safe partition-level bucket counts

[dwangatt]

Summary

This PR is extracted from the original larger PR #9370 and contains the end-to-end implementation required to make partition-level bucket counts safe and usable in Flink.

This PR delivers end-to-end support for partition-level bucket counts in Flink.

It supersedes the closed core-only prerequisite PR #10170. The partition-layout scan prerequisite has already merged in #10052; this PR combines the remaining core routing contract with the supported engine path, Spark policy, documentation, and integration coverage.

Behavior

When bucket.per-partition-count-enabled = true on a partitioned fixed-bucket table:

• Flink loads the active partition-to-bucket mapping when the job starts.
• Flink routes each row with that partition’s bucket count and propagates the same totalBuckets value to the writer.
• The writer validates its routing layout against restored files. A stale streaming job that continues after a partition rescale fails before it can silently write incorrectly routed data.
• INSERT OVERWRITE routes with the target layout so a rescaled partition’s rewritten files are both hashed and stamped with the new bucket count.
• Legacy core writes that only provide (partition, bucket) are rejected when the option is enabled, because they cannot prove which bucket count was used for routing.
• Spark rejects writes through both V1 and V2 write paths; Flink is the supported engine for this option.

The option remains a no-op for unpartitioned tables, which retain the existing single table-level bucket-count invariant.

Documentation

• Adds bucket.per-partition-count-enabled to generated core configuration documentation.
• Documents independent partition rescaling, Flink-only write support, Spark rejection, and the required streaming workflow:
  savepoint stop → rescale/overwrite → restart.

Tests

• Core mapping, extractor, writer restore, and commit validation coverage.
• Flink partitioned PK rescale → overwrite → subsequent write/read coverage.
• Flink streaming savepoint restart after a partition rescale, including a duplicate-key regression assertion.
• Spark write rejection with both spark.paimon.write.use-v2-write=false and true.

Verification

mvn -q -pl paimon-flink/paimon-flink-common -am \
  -Dtest=RescaleBucketITCase -DfailIfNoTests=false test

mvn -q -pl paimon-core \
  -Dtest=PartitionBucketMappingTest,FixedBucketWriteSelectorTest,\
FixedBucketRowKeyExtractorTest,FileSystemWriteRestoreTest,\
FileStoreCommitTest,AppendOnlySimpleTableTest test

mvn -q -pl paimon-spark/paimon-spark-ut -am \
  -Dtest=PaimonSinkTest -DfailIfNoTests=false test