[sundy-li]

Summary

• add PrimitiveType::Variant serde/display support for Iceberg v3 metadata
• cover variant fields in primitive type and V3 table metadata serde tests
• allow Arrow schema conversion for Variant metadata using a binary placeholder so tables with Variant columns can load schema
• keep Avro/value conversions for Variant explicitly unsupported, so actual Variant value read/write support is not implied

Behavior

This PR only makes Iceberg metadata/schema containing "variant" parseable. It does not implement Iceberg Variant value decoding. Consumers should return a clear unsupported error when a query tries to read Variant values.

Validation

• cargo fmt --all --check
• path-crate validation with the Avia metadata sample: serde_json::from_str::iceberg::spec::TableMetadata returns ok
• path-crate validation with the Avia metadata sample: schema_to_arrow_schema(metadata.current_schema()) returns ok with 40 fields

Note: workspace cargo test is still blocked in this environment by crates.io/mockito resolution timeouts, same as the previous validation run.