fix: build_fallback_field_id_map produces incorrect column indices for schemas with nested types

[mbutrovich]

Which issue does this PR close?

• Closes bug: build_fallback_field_id_map produces incorrect column indices for schemas with nested types #2306.
• Downstream issue: bug: native Iceberg reader errors on residual filter on column after nested type for migrated Parquet files datafusion-comet#3860

What changes are included in this PR?

build_fallback_field_id_map iterated over Parquet leaf columns instead of top-level fields when building the field ID to column index mapping for migrated files (no embedded field IDs). When nested types (struct, list, map) precede a primitive column, they expand into multiple leaves, causing the mapping to diverge from add_fallback_field_ids_to_arrow_schema which correctly assigns ordinal IDs to top-level Arrow fields. This made predicates on columns after nested types resolve to a leaf inside the group, crashing with "Leaf column id in predicates isn't a root column in Parquet schema".

The fix iterates root_schema().get_fields() directly, assigning ordinal IDs only to top-level fields. For non-primitive fields (struct/list/map), it uses get_column_root_idx to advance past their leaf columns. This mirrors iceberg-java's ParquetSchemaUtil.addFallbackIds(), which iterates fileSchema.getFields() assigning ordinal IDs to top-level fields.

Also renames "Leave column" to "Leaf column" in error messages.

Are these changes tested?

• An integration test (test_predicate_on_migrated_file_with_nested_types) writes a Parquet file without field IDs containing struct, list, and map columns before an id column, then reads with a predicate on id. This reproduces the exact crash before the fix. Test data is constructed with serde_arrow for readability.

• Apache DataFusion Comet used the repro test in
  apache/datafusion-comet#3860 and it passes with this change:
  test: [DO NOT MERGE] test upstream iceberg-rust fix for #3860 datafusion-comet#3872

[blackmwk]

Thanks @mbutrovich for this fix! Generally LGTM!

[mbutrovich]

Most recent changes:

• crates/iceberg/Cargo.toml — added serde_arrow = { version = "0.14", features = ["arrow-58"] } dev-dependency
• crates/iceberg/src/arrow/reader.rs — replaced three separate tests (test_predicate_on_migrated_file_with_{struct,list,map}) and the shared helper with one test_predicate_on_migrated_file_with_nested_types that:
  • Uses serde_arrow with Serialize/Deserialize structs for readable data construction
  • Tests a schema with all three nested types (struct, list, map) before the id column