perf: compare nested ScalarValues by type before converting to ArrayData

[alexy]

Which issue does this PR close?

• Closes Slow physical planning with many typed NULL struct literals: ScalarValue equality converts nested arrays to ArrayData #26065.

Rationale for this change

Physical planning slows down, faster than the query grows, when projections carry many typed NULL struct literals: a UNION ALL of 35 branches with 35 struct columns of 20 fields takes 862 ms in create_physical_plan on main. EquivalenceGroup::add_constant compares each new constant with every existing class, and ScalarValue::eq compares nested values with arrow's array equality, which converts both arrays to ArrayData before it compares anything, even when the two values have different types.

What changes are included in this PR?

ScalarValue::eq compares the nested variants (List, LargeList, FixedSizeList, ListView, LargeListView, Struct, Map) through a helper that returns early for the same Arc, and compares lengths and data types before arrow's array equality. Results are unchanged; only values of the same type and length reach the ArrayData comparison.

With the reproducer from the issue (release build, Apple M1 Max), create_physical_plan:

struct columns x fields	main	this PR
10 x 20	31 ms	17 ms
20 x 20	179 ms	47 ms
35 x 20	862 ms	147 ms
35 x 40	1,766 ms	253 ms

What is the testing strategy for this PR?

test_nested_scalar_eq covers typed NULL structs of the same and of different types, struct values that are equal and unequal, and lists of different element types. The scalar tests in datafusion-common pass.

Are there any user-facing changes?

No.

🤖 Generated with Claude Code

[adriangb]

THanks!

[adriangb]

I think we should move // manual implementation of PartialEq` to where it was originally

[alamb]

Thank you @alexy

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.83%. Comparing base (b40d696) to head (6a218f4).
⚠️ Report is 108 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26066      +/-   ##
==========================================
+ Coverage   82.69%   82.83%   +0.14%     
==========================================
  Files        1147     1149       +2     
  Lines      447204   452843    +5639     
  Branches   447204   452843    +5639     
==========================================
+ Hits       369793   375134    +5341     
+ Misses      55001    54882     -119     
- Partials    22410    22827     +417     

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