[I] datafusion: relax identity grouping-key gate across partition-spec evolution [iceberg-rust]

["toutane (via GitHub)" <gi...@apache.org>]

toutane opened a new issue, #2658:
URL: https://github.com/apache/iceberg-rust/issues/2658

   ### Is your feature request related to a problem or challenge?
   
   ### Background
   
   `compute_identity_cols` in `crates/integrations/datafusion/src/table/bucketing.rs` (added in #2298) returns `None` which forces the eager scan to declare `UnknownPartitioning` whenever a table has more than one historical partition spec.
   
   This is safe but stricter than iceberg-java, which intersects the identity fields present across all specs (`Partitioning.groupingKeyType` / `commonActiveFieldIds`) and still reports a grouping key on the columns that are identity-partitioned in *every* spec.
   
   ### Why it's conservative today
   
   The eager bucketing path hashes each task on the partition-tuple slot that matches the table's **default** spec. Under spec evolution, older files carry a partition tuple whose slot order does not necessarily align with the default spec, and `FileScanTask` does not currently carry its own spec id to disambiguate. A per-column intersection was attempted in e0d6add and reverted in f25c911 as out of scope for #2298.
   
   ### Describe the solution you'd like
   
   Match iceberg-java: compute the intersection of identity-source fields common to every spec and declare `Partitioning::Hash` on those columns, resolving each task's partition slot via its own spec id rather than assuming the default spec's slot order.
   
   Follow-up to #2298.
   
   ### Willingness to contribute
   
   I would be willing to contribute to this feature with guidance from the Iceberg Rust community


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