feat: add row-count-aware accumulator interfaces

[wudidapaopao]

Which issue does this PR close?

• Closes Support inputless aggregate UDFs with explicit row counts #25885.

Rationale for this change

Aggregate UDFs without arguments receive an empty array slice, so accumulators cannot derive the input row count from their arguments.

What changes are included in this PR?

• Adds argument structs and default accumulator methods that carry evaluated arguments with an explicit row count.
• Adapts GroupsAccumulatorAdapter to pass per-group row counts, including FILTER and state conversion.
• Propagates row counts through physical aggregate execution while preserving existing UDAF behavior.
• This PR supports native Rust UDAFs; forwarding the new row-count context across FFI is left for follow-up work.

What is the testing strategy for this PR?

Added tests for ungrouped inputless aggregation, grouped Adapter execution, FILTER handling, empty state conversion, partial-skip, nullary signature validation, and the unsupported inputless window path.

Are there any user-facing changes?

Adds optional, defaulted public accumulator methods. Existing UDAF implementations remain source-compatible.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.54292% with 58 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.63%. Comparing base (b9c8b3d) to head (fd65f12).
⚠️ Report is 54 commits behind head on main.

Files with missing lines	Patch %	Lines
...gregate-common/src/aggregate/groups_accumulator.rs	84.77%	23 Missing and 7 partials ⚠️
datafusion/physical-plan/src/aggregates/mod.rs	80.00%	22 Missing and 5 partials ⚠️
...n/physical-plan/src/aggregates/aggregate_stream.rs	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25886      +/-   ##
==========================================
+ Coverage   82.57%   82.63%   +0.05%     
==========================================
  Files        1142     1147       +5     
  Lines      441215   445700    +4485     
  Branches   441215   445700    +4485     
==========================================
+ Hits       364332   368282    +3950     
- Misses      54841    55053     +212     
- Partials    22042    22365     +323     

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[mhilton]

Thanks for breaking this PR out. It makes sense to me and I prefer the interface you've added here.

[mhilton]

I wonder if using true_count() would be mildly speedier here.

[wudidapaopao]

Thanks! Updated to use true_count().

[mhilton]

I suppose this does technically invalidate https://datafusion.apache.org/user-guide/sql/window_functions.html#aggregate-functions. I'm not sure that is enough to require making this work in this PR though.