fix(cudf): fall back for unsupported partial identity aggs

[winningsix]

Summary

The TPC-H Q18 warm-run evidence enables cudf.partial_identity_aggregation, but the query includes an aggregate shape that the cuDF partial-identity path does not support. The current code aborts during CudfGroupby::initialize() instead of using the existing regular partial-groupby path.

This PR:

• validates partial-identity capability after the streaming gate and aggregators are built;
• disables the optional partial-identity optimization when any aggregate is unsupported; and
• adds a regression test covering a partial aggregation with sum(c1) and count(0) while the option is enabled.

This preserves correctness and makes the optimization opportunistic rather than fatal. The Spark-side Q21 planning fix is in NVIDIA/spark-gluten#100.

Validation

• git diff --check: PASS
• Focused GPU/CUDA unit test added: AggregationTest.partialIdentityAggregationFallsBackForUnsupportedAggs
• Full GPU test execution was not available in this environment: Docker daemon unavailable and the exact-dev native build is blocked by missing Apache Thrift static libraries.