Parquet: Fix Variant conversion in default Avro writer

[2dmurali]

Problem

The default Parquet Avro writer registers VariantConversion, but conversion lookup does not recognize implementations of the Variant interface.

Values created using Variant.of(...) have the runtime type VariantData. Avro's exact-class conversion lookup therefore misses the conversion registered for Variant.class and attempts to handle the value as an IndexedRecord, causing a ClassCastException.

Changes

• Use VariantConversion for logical Variant values whose runtime classes implement Variant.
• Add a regression test that writes a GenericData.Record through the default Parquet writer and verifies the Variant value after reading it back.

Testing

• ./gradlew :iceberg-parquet:test
• ./gradlew spotlessCheck
• ./gradlew build -x test -x integrationTest
• Manually reproduced the failure and verified the fix using the Iceberg Spark 4.0 runtime.

Closes #17943

[2dmurali]

Hi @nssalian could you take a look at this PR? I noticed you've been actively driving the Variant type feature, so I'd really appreciate your thoughts and review when you have a chance. Thanks!

[uros-b]

Fix seems very scoped, and makes sense to me! Thank you @2dmurali, definitely ping relevant committers who have more context here

[nssalian]

Thanks for the PR @2dmurali. LGTM. I reverted the fix locally and confirmed the test fails with the ClassCastException regression, and passes with the fix. I'll tag folks who can merge this.