feat: rewrite count(non-nullable column) to count(1)

[wudidapaopao]

Which issue does this PR close?

• Closes Rewrite COUNT over non-nullable columns to COUNT(1) #25989.

Rationale for this change

COUNT(non_nullable_column) counts every input row, but evaluating it still reads and materializes the column unnecessarily.

What changes are included in this PR?

• Rewrite safe, non-DISTINCT counts over non-nullable columns to COUNT(1) during expression simplification while preserving output names and schemas.
• Keep SingleDistinctToGroupBy rewrites working when name preservation adds an alias around the rewritten count.
• Avoid marking unchanged aggregate simplifications as transformed.

What is the testing strategy for this PR?

Covered by unit, optimizer, column-pruning, and sqllogictest cases.

Are there any user-facing changes?

Query results and output schemas are unchanged. Optimized plans can show count(1) AS count(column), and scans can prune non-nullable columns used only by COUNT.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.55224% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.62%. Comparing base (b550f59) to head (92fe1b3).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/functions-aggregate/src/count.rs	87.50%	2 Missing and 5 partials ⚠️
...fusion/optimizer/src/single_distinct_to_groupby.rs	86.04%	2 Missing and 4 partials ⚠️
...imizer/src/simplify_expressions/expr_simplifier.rs	97.05%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25990      +/-   ##
==========================================
- Coverage   82.62%   82.62%   -0.01%     
==========================================
  Files        1147     1147              
  Lines      445272   445397     +125     
  Branches   445272   445397     +125     
==========================================
+ Hits       367913   368013     +100     
- Misses      55016    55028      +12     
- Partials    22343    22356      +13     

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[wudidapaopao]

@neilconway Thanks for the suggestion! I split the COUNT(non-nullable column) -> COUNT(1) rewrite out of #25849 into this PR.