[Variant] Support native is_variant_null and Spark 4.2 is_valid_variant #5429

Implement native is_variant_null (Spark 4.0+) and is_valid_variant (Spark 4.2). The whole-value input foundation landed in #5868.

Required behavior

Use Spark's null predicate/helper and validity predicate as the oracle.

Implementation and completion

Add expression-specific serializers/evaluators over [value, metadata]. Validate both components for is_valid_variant and register it only in the Spark 4.2 shim.

Parity and native-plan tests must cover scalar/container kinds, both null forms, malformed value and metadata bytes, and version-specific registration. Keep unrelated Variant operators on fallback and Spark 3 compilation unchanged.

Parent: #5438. This is the first native expression slice before #5424/#5425.