Parquet: Default Avro writer fails to write Variant values

### Apache Iceberg version

1.11.0 (latest release)

### Query engine

Spark

### Please describe the bug 🐞

### Description

The default Iceberg Parquet Avro writer fails when writing a `GenericData.Record` containing a logical `Variant` value.

`Parquet.write(...).schema(schema).build()` uses `ParquetAvro.DEFAULT_MODEL`, which registers `VariantConversion`. However, Avro looks up conversions using the datum's exact runtime class. Values returned by `Variant.of(...)` implement `Variant`, but their runtime class is `VariantData`. Because `VariantConversion#getConvertedType()` returns `Variant.class`, the conversion is not found.

Avro consequently treats the value as an `IndexedRecord`, resulting in a `ClassCastException`.

### Reproduction

```java
Schema schema = new Schema(required(1, "v", Types.VariantType.get()));

GenericData.Record record =
new GenericData.Record(AvroSchemaUtil.convert(schema.asStruct(), "table"));
record.put("v", Variant.of(Variants.emptyMetadata(), Variants.of(34)));

try (FileAppender writer =
Parquet.write(Files.localOutput(file)).schema(schema).build()) {
writer.add(record);
}
```

### Actual behavior

Writing the record fails because `VariantData` cannot be cast to Avro's `IndexedRecord`.

### Expected behavior

The registered `VariantConversion` should handle any runtime class implementing `Variant`, allowing the value to be written successfully.

### Willingness to contribute

- [x] I can contribute a fix for this bug independently
- [x] I would be willing to contribute a fix for this bug with guidance from the Iceberg community
- [ ] I cannot contribute a fix for this bug at this time

Start by running the supplied GenericData.Record reproduction through Parquet.write(...).schema(schema).build(), then inspect ParquetAvro.DEFAULT_MODEL and VariantConversion#getConvertedType(). Done means the registered conversion handles the runtime VariantData value and the record writes successfully without the IndexedRecord ClassCastException.

Written by the indexing model from the issue text.