perf: cache dictionary arc pointer

[Rich-T-kid]

Which issue does this PR close?

• Closes cache dictionary arc ptr in groupColumn #23921.
• this PR was created on top of introduce new dictionary benchmarks #24416, so once that is merged in we can rebase this branch

Rationale for this change

append_val is called once per new group on the scalar (streaming) code path. Previously it hashed a single-element slice of the dictionary values array on every call, paying create_hashes fixed overhead 65k times for a high-cardinality batch. Caching the full values array hash keyed on Arc::ptr_eq collapses that to one vectorized hash pass per batch, yielding a 2× speedup on the all-unique case with no measurable regression elsewhere.

What changes are included in this PR?

• DictionaryGroupValuesColumn gains a cached_values: Option field
• append_val: on a cache miss (Arc::ptr_eq fails), hashes the entire dict.values() array into val_hashes and stores the Arc; on a hit, reuses val_hashes[val_idx] directly — eliminating the per-call slice(val_idx, 1) allocation and create_hashes call

Are these changes tested?

nothing logically changed.

Are there any user-facing changes?

no

[Rich-T-kid]

cc @zhuqi-lucas since this was a follow up you mentioned 😄

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.23%. Comparing base (bbf739a) to head (4a7a664).

Files with missing lines	Patch %	Lines
...gregates/group_values/multi_group_by/dictionary.rs	94.44%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24418      +/-   ##
==========================================
- Coverage   81.23%   81.23%   -0.01%     
==========================================
  Files        1111     1111              
  Lines      390208   390223      +15     
  Branches   390208   390223      +15     
==========================================
+ Hits       316990   316995       +5     
- Misses      54591    54597       +6     
- Partials    18627    18631       +4     

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