Parquet: Fix NPE in ParquetAvroWriter when schema contains variant type

[nssalian]

Summary

Fixes: #15933

ParquetAvroWriter.WriteBuilder inherits ParquetTypeVisitor.variant() which returns null. For OPTIONAL variant fields, struct() wraps this null in OptionWriter, which dereferences writer.columns() resulting in an NPE

Changes

Override variant() to throw UnsupportedOperationException, similar to primitive() handles unsupported types in the same class.

Test plan

• Wrote a test to check the UnsupportedOperationException and ran it locally.
• Removed the fix in ParquetAvroWriter and verified the NPE is reproducible.

[nssalian]

Fixes for the runtime deps and zizmor is #15937. That would need to be merged and I'll rebase after it to get the tests to pass.

[nssalian]

CC: @manuzhang @aihuaxu