Support StringDecode for GBK encoding

[thirtiseven]

Fixes #14542

Description

This PR adds support for StringDecode on GBK encoding.

Depends on NVIDIA/spark-rapids-jni#4431

performance testing:

=== GPU (spark-rapids) ===
Warmup (3 runs)...
Benchmark (5 runs)...
  GPU     min=1048 ms  median=1068 ms  avg=1075 ms  max=1101 ms

=== CPU (Spark Java) ===
Warmup (3 runs)...
Benchmark (5 runs)...
  CPU     min=13526 ms  median=13791 ms  avg=13766 ms  max=13984 ms

=== Speedup: 12.9x (CPU median / GPU median) ===

bench_gbk_decode.scala.zip

Checklists

• This PR has added documentation for new or modified features or behaviors.
• This PR has added new tests or modified existing tests to cover new code paths.
  (Please explain in the PR description how the new code paths are tested, such as names of the new/existing tests that cover them.)
• Performance testing has been performed and its results are added in the PR description. Or, an issue has been filed with a link in the PR description.

[thirtiseven]

@greptile review

[greptile-apps]

Greptile Summary

This PR adds GPU-accelerated decode(bin, charset) for GBK encoding (~13× speedup over CPU). Pre-Spark 4.0 versions use a new BinaryExprMeta[StringDecode] shim, while Spark 4.0+ routes through a new StaticInvokeMeta in objectsExpressions.scala since Spark replaces StringDecode with a StaticInvoke during analysis. Non-GBK charsets correctly fall back to CPU via willNotWorkOnGpu.

Confidence Score: 5/5

Safe to merge; implementation is correct and consistent with existing patterns

All remaining findings are P2. The charsetName null concern is a defensive coding preference — the framework guarantees convertToGpuImpl is never called after willNotWorkOnGpu. The prior thread's argument bounds-check concern has been addressed with the arguments.size < 2 guard at line 59 of objectsExpressions.scala.

No files require special attention

Important Files Changed

Filename	Overview
sql-plugin/src/main/scala/com/nvidia/spark/rapids/objectsExpressions.scala	New StaticInvokeMeta for Spark 4.0+ StringDecode via StaticInvoke; charsetName stays null if willNotWorkOnGpu fires, though the framework prevents convertToGpuImpl from running in that case
sql-plugin/src/main/scala/org/apache/spark/sql/rapids/stringFunctions.scala	GpuStringDecode case class correctly delegates to CharsetDecode.decode JNI for GBK; NullIntolerantShim handles null input propagation
sql-plugin/src/main/spark321/scala/com/nvidia/spark/rapids/shims/StringDecodeShims.scala	Pre-Spark 4.0 StringDecode shimmed as BinaryExprMeta with GBK-only GPU support; shim-json annotation covers all 321–358 versions including Databricks variants
sql-plugin/src/main/spark400/scala/com/nvidia/spark/rapids/shims/StringDecodeShims.scala	Spark 4.0+ path correctly returns an empty map; StringDecode is handled via StaticInvokeMeta instead due to Spark's RuntimeReplaceable rewrite
integration_tests/src/main/python/string_test.py	Adds basic (fixed byte sequences, nulls, invalid bytes) and random GBK decode tests; random test uses the framework's global datagen seed for reproducibility
sql-plugin/src/main/scala/com/nvidia/spark/rapids/GpuOverrides.scala	Wires StringDecodeShims.exprs into the expressions map alongside existing shim registrations

Sequence Diagram

sequenceDiagram
    participant Spark as Spark Catalyst
    participant ShimMeta as StringDecodeShims/StaticInvokeMeta
    participant GpuExpr as GpuStringDecode
    participant JNI as CharsetDecode (JNI)

    Spark->>ShimMeta: tagExprForGpu()
    ShimMeta->>ShimMeta: extractLit(charset) == GBK?
    alt GBK literal
        ShimMeta->>ShimMeta: charsetName = "GBK"
        ShimMeta->>GpuExpr: convertToGpu() → GpuStringDecode(bin, "GBK")
        GpuExpr->>JNI: CharsetDecode.decode(column, GBK)
        JNI-->>GpuExpr: UTF-8 ColumnVector
    else non-GBK or non-literal
        ShimMeta->>Spark: willNotWorkOnGpu → CPU fallback
    end

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[thirtiseven]

@greptile review