chore: remove dead useDecimal128 plumbing

[andygrove]

Which issue does this PR close?

Closes #.

Rationale for this change

The internal spark.comet.use.decimal128 conf is dead plumbing:

• The conf is write-only: three callers (CometExecRule, CometShuffleExchangeExec, CometNativeExec.doExecuteColumnar) force it to "true" right before native execution, but nothing reads it anywhere in the repo.
• The useDecimal128 boolean threaded through the entire CometVector hierarchy is always true in production. Both NativeUtil.importVector and NativeUtil.rootAsBatch hardcode true, so the !useDecimal128 branch in CometVector.getDecimal() is unreachable.
• The Rust SparkParquetOptions.use_decimal_128 field is declared, defaulted to false in both ::new constructors, and never set or read elsewhere.

This looks like legacy from the era when the JVM Parquet reader produced Decimal32/Decimal64 vectors. With the native scan path being the only path, every decimal column already lands in 128-bit form before reaching the JVM.

What changes are included in this PR?

• Remove CometConf.COMET_USE_DECIMAL_128 and its three setConfString writers.
• Remove the useDecimal128 field and constructor parameter from CometVector, CometDecodedVector, CometPlainVector, CometDictionaryVector, CometListVector, CometMapVector, CometStructVector, CometDelegateVector, CometSelectionVector.
• Collapse CometVector.getDecimal() to the always-128-bit path.
• Drop the true arg from both NativeUtil callers of CometVector.getVector and from the codegen template in CometBatchKernelCodegenInput.
• Remove use_decimal_128 from SparkParquetOptions and clean up three stale comments in columnar_to_row.rs that referenced the conf.
• Update TestColumnReader callers to the simplified constructor.

How are these changes tested?

Covered by existing tests:

• ParquetReadV1Suite "decimals": passes (Standard + Legacy modes plus sibling decimal tests).
• CometShuffleSuite "fix: StreamReader should read shuffled decimal columns as Decimal128": passes (renamed from the old useDecimal128 regression test, still pins NativeUtil.rootAsBatch against decimal-128 expectation).
• The Seq(true, false).foreach { useDecimal128 => ... } sweep in ParquetReadSuite "decimals" was flattened since the conf no longer exists; the remaining batch-size and precision matrix still exercises the decimal read path.
• make and cargo clippy --all-targets --workspace -- -D warnings both green.

[mbutrovich]

Thanks for finding the dead code, @andygrove!