Support Spark 4.2 insert-only MERGE execution #6612

Problem

Spark 4.2 introduces InsertOnlyMergeExec for insert-only MERGE plans. These plans can bypass the normal MergeRowsExec path, so native MergeRows support alone does not cover all MERGE shapes.

Proposed solution

Support Spark 4.2 insert-only MERGE while preserving Spark's V2 MergeSummary contract:

• support multiple NOT MATCHED clauses through native MergeRows where applicable
• keep InsertOnlyMergeExec responsible for its writer summary semantics
• preserve insert-only behavior without cardinality validation
• cover the single-clause optimized path that has no MergeRowsExec child

Add result and MergeSummary parity tests against Spark.

Related: #5318, #5122