fix: allow name mappings without names

[aarushigupta132]

Summary

• accept Java-compatible name mapping entries that omit names by defaulting to an empty list
• add direct deserialization and round-trip coverage while retaining explicit names

Testing

• cargo fmt --all -- --check
• cargo test -p iceberg spec::name_mapping --lib
• cargo clippy -p iceberg --lib --tests -- -D warnings
• cargo test -p iceberg --lib