refactor(spec): make UnboundPartitionField fields private and carry source-ids

[moomindani]

Which issue does this PR close?

Part of #3172 and #2801. First of the three per-struct PRs @blackmwk asked for in #2802, starting with UnboundPartitionField as he suggested. #2802 stays open and will be narrowed to PartitionField once this lands.

What changes are included in this PR?

UnboundPartitionField gets the #3172 treatment — private fields with accessors — and with it the v3 source-ids spelling.

• The four fields are private, with source_id() -> Result<i32>, source_ids() -> &[i32], field_id(), name() and transform() accessors. The derived TypedBuilder is kept but its visibility is limited, so an instance can only be built inside the crate or read out of a spec JSON.
• source_ids: Vec<i32> replaces source_id: i32 and always holds at least one id. source_id() returns the single id, and an error for a multi-argument field rather than quietly handing back the first one.
• A _serde module reads either spelling — source-id, or source-ids for a v3 multi-argument transform — and writes back the one that matches the field. An empty source-ids, a missing id, and a source-id that disagrees with the first source-ids entry are all rejected.
• Binding a multi-argument field now fails with a clear error. A bound PartitionField still carries a single source_id, so until that struct is converted there is nowhere to put the extra ids, and failing loudly beats dropping them on the way into TableCreation or TableUpdate::AddSpec.
• check_for_redundant_partitions compares the whole id list instead of one id.

Three derived decisions worth your call:

• The builder is pub(crate) rather than module-private, because table_metadata_builder, partitioning and the expression visitors all construct these. Say the word if you want it tighter and I will route those through UnboundPartitionSpecBuilder.
• add_unbound_field and add_unbound_fields stay public, but outside the crate they can now only be fed a field cloned out of an existing spec. Happy to make them pub(crate) if that is the intent.
• The field-id reuse path in table_metadata_builder mutated field_id in place, which private fields no longer allow, so there is a crate-internal with_field_id.

Not in this PR, and not lost: PartitionField (#2802, to be narrowed once this lands) and SortField get the same treatment, and with them the multi-argument read support and the Transform::Unknown mapping that #2801 is about.

Are these changes tested?

Yes — four new unit tests in spec/partition.rs: reading a source-ids-only field and writing it back, the single-id round trip, the three malformed-id rejections, and binding a multi-argument field failing loudly. cargo test -p iceberg --lib passes (1768 tests), with cargo fmt, cargo clippy --workspace --all-targets --all-features -- -D warnings and a full workspace build clean. crates/iceberg/public-api.txt is regenerated: four public fields and the public builder() out, five accessors in.

AI Disclosure

This pull request and its description were written by Isaac.