fix(spec): reject unequal map default arrays

[mattfaltyn]

Which issue does this PR close?

• Closes bug(spec): unequal map default arrays are silently truncated #3109.

What changes are included in this PR?

• Reject map literals whose parallel keys and values arrays have different lengths instead of silently truncating them with Iterator::zip.
• Propagate invalid field-default parse errors through NestedField deserialization so malformed metadata cannot be accepted as a missing default.
• Cover both mismatch directions at the literal boundary and both initial-default and write-default at the schema boundary.

This matches the Java reference implementation's equal-length requirement for map literals.

Are these changes tested?

Yes.

• cargo +stable test -p iceberg --lib spec::values::tests::json_map_rejects_mismatched_key_value_lengths -- --exact --nocapture
• cargo +stable test -p iceberg --lib spec::datatypes::tests::nested_field_rejects_invalid_map_defaults -- --exact --nocapture
• make check
• make build
• make unit-test
• cargo nextest run --all-targets --all-features --workspace --no-fail-fast: 2,107 passed; the only 20 failures were HMS integration cases because the pinned amd64-only Hive 3.1.3 container did not bind port 9083 under ARM64 emulation on this host.
• cargo nextest run --all-targets --all-features --workspace -E 'not (package(iceberg-catalog-hms) | test(/hms_catalog/))': 2,090/2,090 passed, including REST, Glue, S3Tables, SQL, DataFusion, Spark, S3, GCS, and Hugging Face integrations. HMS unit tests passed under make unit-test.

AI Disclosure

I used Codex to assist with repository inspection, implementation, test execution, and drafting this description. I reviewed the bug reproduction, diff, and test results before submission.