[Variant] Support VariantType in native columnar-to-row conversion

### What is the problem the feature request solves?

Comet keeps native columnar-to-row conversion on Spark fallback when a schema contains `VariantType`. This prevents a native scan or projection that carries Variant from feeding row-based Spark operators through Comet's native C2R path.

Spark does not encode Variant in `UnsafeRow` as an ordinary nested Struct. [`VariantVal`](https://github.com/apache/spark/blob/v4.1.3/common/unsafe/src/main/java/org/apache/spark/unsafe/types/VariantVal.java#L28-L46) defines a dedicated payload, and [`UnsafeWriter.write(VariantVal)`](https://github.com/apache/spark/blob/v4.1.3/sql/catalyst/src/main/java/org/apache/spark/sql/catalyst/expressions/codegen/UnsafeWriter.java#L164-L182) writes `4-byte value length + value bytes + metadata bytes`.

Comet's current [`supportsSchema`](https://github.com/peterxcli/datafusion-comet/blob/c355fefd9c0b7e96d86523a7214bb2cdd47e1a55/spark/src/main/scala/org/apache/spark/sql/comet/CometNativeColumnarToRowExec.scala#L230-L259) does not admit Variant, while the native [C2R initialization path](https://github.com/peterxcli/datafusion-comet/blob/c355fefd9c0b7e96d86523a7214bb2cdd47e1a55/native/core/src/execution/jni_api.rs#L1018-L1034) currently derives only the physical Arrow datatype. Treating the `[value, metadata]` storage as a normal Struct would produce the wrong UnsafeRow representation.

### Describe the potential solution

- Preserve explicit Variant identity when initializing native C2R.
- Add a top-level Variant writer that consumes Comet's canonical `[value: Binary, metadata: Binary]` array and emits Spark's exact UnsafeRow Variant payload.
- Preserve the distinction between SQL NULL and Variant JSON null, and validate malformed child/null combinations rather than panicking.
- Keep Variant out of generic Struct handling.

Focused parity tests should cover objects, arrays/scalars, Variant JSON null, SQL NULL, nullable parents, empty batches, and fields before/after Variant. The resulting row should expose Spark `VariantType` and round-trip through `UnsafeRow.getVariant` with the same value and metadata bytes.

### Additional context

Related work: #5407 establishes the native columnar `[value, metadata]` representation; #5425 tracks Variant-valued native expression output. This issue is only C2R transport.

Nested Variant, shuffle/spill, Python transport, writes, and general Variant expressions remain separate. Spark 3.x behavior must remain unchanged.