perf: avoid FFI import/export between native subtree and ShuffleWriter

[mbutrovich]

Which issue does this PR close?

Closes #3925. Replaces #3930.

Rationale for this change

When CometShuffleExchangeExec has a CometNativeExec child, the current code runs two CometExecIterators per partition: one for the upstream subtree, one for Scan("ShuffleWriterInput") -> ShuffleWriter consuming the FFI'd batches back. The JVM doesn't read the data, so the round-trip is pure overhead.

This PR collapses the two iterators into one CometExecIterator rooted at ShuffleWriter(child = childNativeOp): no JVM intermediary, no Arrow FFI export/import, one fewer createPlan/releasePlan pair per partition. The proto already supported this nested form.

Why this replaces #3930

#3930 also removed the FFI but kept the two-iterator structure, passing batches between them through a JVM-side u64-keyed Mutex<HashMap<u64, RecordBatch>>. This PR achieves the same result without the global registry, the batch-handle protocol, or the second iterator: the child plan is just a child operator in the same DataFusion plan as the writer.

What changes are included in this PR?

• CometNativeExec exposes NativeExecContext (buildNativeContext() / executeColumnarWithContext(ctx)) so the shuffle path reuses doExecuteColumnar's per-partition setup (broadcast alignment, plan-data injection, subqueries, encryption).
• CometNativeShuffleInputRDD is a thin scheduling-anchor RDD that resolves leaf partitions on the driver and carries per-partition leaf/shuffle-block iterators.
• NativeShuffleSpec (child native op, metric node, exec context) is carried by CometShuffleDependency and only populated on native-child shuffles.
• CometShuffleExchangeExec dispatches: native-child shuffles use prepareNativeShuffleDependency and the thin RDD; everything else uses the existing 5-arg prepareShuffleDependency with a synthesized Scan("ShuffleWriterInput") placeholder. Behavior unchanged for CometSparkToColumnarExec, CometCollectLimitExec, CometTakeOrderedAndProjectExec.
• CometNativeShuffleWriter rewritten to one code path over ShuffleWriter(child = spec.childNativeOp).

Removing the FFI boundary surfaces three issues the old deep-copy + ScanExec cast was masking:

• DataFusion-vs-catalyst Arrow type drift (width_bucket, date_trunc, collect_set, ...). New SchemaAlignExec between the inlined child and the writer casts per column and widens nullability to actual.nullable || expected.nullable. Each distinct drift logs one process-deduped warning pointing at DataFusion / DataFusion-Spark functions whose Arrow return type drifts from Spark catalyst's declared type #4515. New ShuffleWriter.expected_output_schema proto field.
• Catalyst-pruned HashAggregateExec.resultExpressions (EXISTS / row-existence-only subqueries) where the JVM-declared output=[] disagrees with the native aggregate's natural shape. Fixed in CometBaseAggregate.doConvert: when natural shape differs from declared shape, emit an explicit Projection proto op above the HashAgg. Native OpStruct::HashAgg always returns its natural shape. HashAggregate.result_exprs (field 3) and apply_result_projection (field 8) reserved.
• Typed exception loss through ShuffleWriterExec. With the writer now the root operator for any stage that ends in a shuffle, an upstream DataFusionError::External(SparkError) (e.g. decimal overflow in ANSI mode) was being re-wrapped by external_shuffle's ArrowError::ExternalError adapter, hiding the typed exception from the JNI bridge's single-level downcast. Drop the wrap; try_flatten already handles the matching DataFusionError types directly.

How are these changes tested?

Existing tests, plus:

• New CometAggregateSuite regression test for the catalyst-pruned aggregate path. The original Spark SQL test that surfaced the aggregate bug (subquery/exists-subquery/exists-orderby-limit.sql query 19) is also covered by the SQL-tests harness CI.