fix: propagate field metadata through coalesce, nvl, and nvl2

[james-willis]

Which issue does this PR close?

• Part of CASE expressions drop field metadata, silently stripping Arrow extension types and breaking metadata-aware UDFs #24860.

Rationale for this change

COALESCE, NVL/IFNULL, and NVL2 drop the field metadata of their arguments when computing their return field: return_field_from_args builds the output Field from the argument data types only. For columns carrying Arrow extension types (ARROW:extension:name / ARROW:extension:metadata — e.g. arrow.uuid, geoarrow.wkb), the planned schema of any expression containing these functions silently loses the extension-type identity, and metadata-aware UDFs that dispatch on their argument Fields fail to plan over them (see #24860 for a full reproducer).

This is one of several independent metadata drop sites listed in #24860. It is the plan-time schema fix for the conditional functions; it does not by itself change execution-time behavior, because these functions simplify into CASE, which has its own drop sites (also #24860) that I intend to address in follow-up PRs.

What changes are included in this PR?

• A small unanimous_metadata helper in datafusion/functions/src/utils.rs: returns the metadata shared by every argument field that can contribute a value to the result. Fields with a Null data type (untyped NULL literals) are ignored; if the remaining fields disagree, the result carries no metadata rather than claiming a type identity that only some inputs have.
• coalesce: return_field_from_args now attaches the unanimous metadata of its arguments. Return type and nullability computation are unchanged. NVL/IFNULL delegate to coalesce and are fixed by the same change.
• nvl2: same, over the second and third arguments only — the first argument is only tested for NULL and never contributes a value, so its metadata is excluded.

What is the testing strategy for this PR?

Unit tests in coalesce.rs and nvl2.rs directly exercise return_field_from_args: metadata propagated when the value arguments agree, dropped when they disagree, untyped NULL arguments not blocking propagation, NVL2's test argument excluded, and nullability unchanged.

There is deliberately no sqllogictest: arrow_metadata() reads its argument field at execution time, and at execution these functions have been simplified into CASE, whose own metadata drops (#24860) still lose the metadata en route. End-to-end SLT coverage lands with the CASE fix. (This ordering is safe: the optimizer's schema invariant intentionally ignores metadata — assert_expected_schema / logically_equivalent_names_and_types — so a plan whose coalesce carries metadata that the simplified CASE does not does not error; it just degrades to today's behavior until the CASE fix lands.)

Are there any user-facing changes?

The planned schema (e.g. DataFrame::schema()) of expressions containing COALESCE/NVL/IFNULL/NVL2 over metadata-bearing columns now preserves that metadata. No API changes.

---

cc @paleolimbot — this is the first slice of #24860, in the same family as your #22112; you'd mentioned on #21984 you could help shepherd this class of metadata fixes. This is my first DataFusion PR, so CI will need a committer to trigger it.

[james-willis]

In this PR, if the branches of the coalesce, etc don't agree on the metadata, we drop extension metadata defensively.

My opinion is that this should actually be a planning time error unless all rows will hit only one branch but wanted to take the more conservative approach for now. I want to solicit opinions from the reviewers on what the desried behavior is here.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.62%. Comparing base (fefc227) to head (b48d6ba).

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #24865    +/-   ##
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  Coverage   81.61%   81.62%            
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  Files        1123     1123            
  Lines      409392   409506   +114     
  Branches   409392   409506   +114     
========================================
+ Hits       334143   334245   +102     
- Misses      55630    55637     +7     
- Partials    19619    19624     +5     

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