bench: add simplify_function_over_case benchmark

[radmirnovii]

Which issue does this PR close?

• Refs Simplifier: push immutable scalar functions into CASE branches with literal outputs #24477.

Rationale for this change

Spin-off of #24477: have a benchmark for f(CASE-of-literals) simplification
and evaluation available before the PR with the rule, to compare against main.

What changes are included in this PR?

A new simplify_function_over_case Criterion benchmark for
datafusion-optimizer: simplify/{2,8,32} (plan-time simplifier cost over N
literal branches) and evaluate_8192_rows (per-batch evaluation after the
standard simplification pipeline). Measurement coverage only; no behavior change.

Are these changes tested?

Yes: the benchmark compiles and runs on main
(cargo bench -p datafusion-optimizer --bench simplify_function_over_case);
fmt and clippy pass.

Are there any user-facing changes?

No.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.39%. Comparing base (6cc3442) to head (c83c2d0).
⚠️ Report is 52 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24489      +/-   ##
==========================================
- Coverage   81.39%   81.39%   -0.01%     
==========================================
  Files        1118     1118              
  Lines      398692   398692              
  Branches   398692   398692              
==========================================
- Hits       324525   324518       -7     
- Misses      55198    55205       +7     
  Partials    18969    18969              

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[kosiew]

@radmirnovii,

Thanks for adding this benchmark ahead of the optimizer change. The setup looks useful for comparing the current behavior against the follow-up rule, and I don't see anything blocking here.

I left a couple of optional suggestions around making the evaluation benchmark a bit easier to interpret and compare.

[radmirnovii]

Thanks for the review.

[kosiew]

🚀
@radmirnovii
Thank you for your contribution.