Kafka Connect: Enable Parquet variant shredding for generic Record writes

[soumilshah1995]

Summary

Kafka Connect uses Iceberg’s generic Record model with a Void engine schema. Parquet variant shredding was ineffective on that path because the generic ParquetFormatModel did not use a variant shredding analyzer / row copier, and RecordVariantShreddingAnalyzer could not resolve VARIANT columns (engine resolveColumnIndex is a dead end for Void).

This PR wires RecordVariantShreddingAnalyzer and Record::copy into GenericFormatModels and analyzes VARIANT columns by Iceberg Schema#columns() order so buffered inference and shredded Parquet columns work for Connect.

Changes

• GenericFormatModels: register ParquetFormatModel with RecordVariantShreddingAnalyzer + Record::copy.
• RecordVariantShreddingAnalyzer: implement analyzeVariantColumns using positional indices aligned with Record#get.

Config (Connect)

Table write properties (e.g. via iceberg.tables.write-props):

• write.parquet.shred-variants=true
• write.parquet.variant-inference-buffer-size=<rows>

Test plan

• ./gradlew :iceberg-data:check :iceberg-kafka-connect:iceberg-kafka-connect:check (or CI green).
• Connect sink writing VARIANT with write.parquet.shred-variants=true; inspect Parquet for typed_value paths / higher physical column count vs false.
• Regression: Connect append with shredding disabled still succeeds.