feat: Native columnar to row conversion (Phase 2)

[andygrove]

Which issue does this PR close?

Part of #3268

Follow on from #3221

Rationale for this change

Enable CometNativeColumnarToRowExec in Comet tests, but still disabled by default.

What changes are included in this PR?

• Mutable buffer fallback - Fall back to JVM C2R for V2 Parquet scans and native_iceberg_compat scans with partition columns that use mutable buffers
• Int32/Int64 → Decimal128 conversion - Handle small-precision decimals stored as Int32/Int64 when COMET_USE_DECIMAL_128=false
• Dictionary-encoded decimal corruption - Fix 2x value multiplication when casting dictionary-encoded decimals
• FixedSizeBinary support - Add missing support for FixedSizeBinary data type
• NullArray handling - Cast NullArray to expected schema type when all column values are null
• Dictionary unpacking - Auto-unpack dictionary-encoded arrays when schema expects non-dictionary type
• Dictionary type mismatch - Fix "Expected Dictionary type but got Binary" error with BloomFilter columns
• doExecuteBroadcast - Add broadcast support to CometNativeColumnarToRowExec
• refactor to avoid duplicate code

How are these changes tested?

[codecov-commenter]

Codecov Report

❌ Patch coverage is 65.88235% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.96%. Comparing base (f09f8af) to head (fa54060).
⚠️ Report is 880 commits behind head on main.

Files with missing lines	Patch %	Lines
...spark/sql/comet/CometNativeColumnarToRowExec.scala	65.27%	20 Missing and 5 partials ⚠️
...he/comet/rules/EliminateRedundantTransitions.scala	75.00%	0 Missing and 3 partials ⚠️
.../scala/org/apache/spark/sql/comet/util/Utils.scala	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3266      +/-   ##
============================================
+ Coverage     56.12%   59.96%   +3.83%     
- Complexity      976     1468     +492     
============================================
  Files           119      175      +56     
  Lines         11743    16149    +4406     
  Branches       2251     2680     +429     
============================================
+ Hits           6591     9683    +3092     
- Misses         4012     5121    +1109     
- Partials       1140     1345     +205     

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[andygrove]

@wForget I've now added the fix for closing the ColumnarBatch after conversion (commit a585707).

The issue was that after calling converter.convert(batch), the batch was not being closed, which could lead to Arrow resource leaks. The fix wraps the row iterator to call batch.close() when hasNext returns false:

new Iterator[InternalRow] {
  override def hasNext: Boolean = {
    val hasMore = result.hasNext
    if (!hasMore) {
      batch.close()
    }
    hasMore
  }
  override def next(): InternalRow = result.next()
}

This fix is applied to both the doExecute() path and the broadcast execution path.