[core] Optimize Flink BTree index topology

[leaves12138]

What changed

• Reworked Flink BTree global index building to use one task-driven topology for all contiguous row ranges instead of building one topology per range.
• Added an internal build task id to the sort key so each range keeps its own row-range metadata while sharing the same Flink source/read/sort/write chain.
• Added coverage for parallelism calculation, many small ranges, and a single large range split across multiple writer subtasks.

Why

When row ranges are highly fragmented, the old implementation creates a separate Flink topology for each range. That can make the create-index procedure spend a long time constructing the JobGraph and can produce an oversized topology.

Validation

• mvn -pl paimon-flink/paimon-flink-common -DfailIfNoTests=false -Dtest=BTreeIndexTopoBuilderTest test
• mvn -pl paimon-flink/paimon-flink-common -Pfast-build -DfailIfNoTests=false -Dtest=BTreeGlobalIndexITCase#testBTreeIndexWithManyPartitions test
• mvn -pl paimon-flink/paimon-flink-common -Pfast-build -DfailIfNoTests=false -Dtest=BTreeGlobalIndexITCase#testBTreeIndexWithSingleRangeAndParallelWriters test