datafusion: relax identity grouping-key gate across partition-spec evolution

### Is your feature request related to a problem or challenge?

Feature request.

### 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

Start in crates/integrations/datafusion/src/table/bucketing.rs at compute_identity_cols and trace the eager bucketing path, then read the context from #2298 and the FileScanTask limitation. Verify how partition specs and identity-source fields are represented before changing the grouping-key decision. Done means common identity fields across all specs produce Partitioning::Hash and each task resolves its partition slot using its own spec id.

Written by the indexing model from the issue text.