introduce new dictionary benchmarks

[Rich-T-kid]

Which issue does this PR close?

• works towards cache dictionary arc ptr in groupColumn #23921 & Optimize take_n on DictionaryGroupValuesColumn<K> #24089.

Rationale for this change

The existing benchmarks only cover the vectorized (GroupOrdering::None) intern/emit path. This adds coverage for the scalar append_val/equal_to path and the rolling EmitTo::First (take-n) emit path.

What changes are included in this PR?

Two new benchmarks in benches/dictionary_group_values.rs:

• bench_scalar_append_equal — uses GroupOrdering::Full to exercise the scalar GroupValuesColumn:: code path
• bench_take_n — uses EmitTo::First(1) in a loop over N_BATCHES iterations to exercise the partial/rolling emit path

Are these changes tested?

n/a

Are there any user-facing changes?

n/a

[Rich-T-kid]

cc @saadtajwar @kumarUjjawal 🚀

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.24%. Comparing base (c7be84a) to head (200910d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24416      +/-   ##
==========================================
- Coverage   81.24%   81.24%   -0.01%     
==========================================
  Files        1112     1112              
  Lines      390912   390912              
  Branches   390912   390912              
==========================================
- Hits       317604   317595       -9     
- Misses      54670    54675       +5     
- Partials    18638    18642       +4     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[saadtajwar]

Nice! Will definitely make sure to use this for #24089