Support native last / last(ignoreNulls) aggregate

[zhuxiangyi]

Background

Auron currently accelerates first / first(ignoreNulls) aggregates natively (AggFirst / AggFirstIgnoresNull), but last / last(ignoreNulls) are not implemented natively. As a result they fall back to the generic UDAF path (a JNI call back into the JVM), losing vectorized acceleration.

Proposal

Add native last aggregate support, mirroring the existing first implementation:

1. Implement AggLast and AggLastIgnoresNull in datafusion-ext-plans with the same columnar accumulator layout as first, but with "later value wins" semantics (every visited row overwrites; on merge the later partial state wins).
2. Wire the two new functions through the AggFunction enum, create_agg, the protobuf contract (LAST / LAST_IGNORES_NULL), the protobuf::AggFunction -> AggFunction conversion, and the window-aggregate mapping.
3. Add the Last expression conversion in NativeConverters.convertAggregateExpr (dispatching to LAST / LAST_IGNORES_NULL by ignoreNulls).
4. Declare the native aggregate buffer schema for Last in NativeAggBase.computeNativeAggBufferDataTypes ([dataType] for ignoreNulls, [dataType, Boolean] otherwise) so the partial -> shuffle -> final buffer schema matches the native side.

Scope / Non-goals

• Covers last(col) and last(col, ignoreNulls = true) over primitive / boolean / string / binary / complex (scalar-value) types, consistent with first.
• Window aggregate (last over a window) reuses the same AggFunction.
• Semantics follow Spark: last is order-sensitive and non-deterministic without an explicit ordering (same as first).

Tests

• Rust unit test agg_exec::test::test_agg_last: partial -> final two-phase aggregation over a nullable column, verifying last (keeps the last visited row including null) and last(ignoreNulls) (keeps the last non-null value).
• Scala end-to-end test in AuronDataFrameAggregateSuite ("native last / last(ignoreNulls) aggregate", spark34 + spark35): a grouped aggregate exercising the full partial -> shuffle -> final native path, asserting correct values and that the plan offloads to NativeAggBase.

[zhuxiangyi]

PR: #2359