fix: Address avg return type overflow for decimals

[AdamGS]

Which issue does this PR close?

• Closes avg(decimal) might lose precision and cause an overflow over return type #24369

Rationale for this change

AVG(Decimal(p, s)) derives a result with precision and scale increased by four. For example, AVG(Decimal32(9, 0)) returned Decimal32(9, 4), although the rescaled average can require 13 digits.

This caused valid averages near the input precision limit to fail with an overflow even though Decimal64(13, 4) can represent the result.

What changes are included in this PR?

• Preserve the input decimal variant when the derived result fits, and widen it when necessary, while still clamping Decimal256 at MAX.
• Allow decimal AVG accumulators to use different native types for the input, intermediate sum, and output.
• Update SQL type assertions and affected logical-plan expectations.

Are these changes tested?

Regression tests and adjusted SLT-based tests.

Are there any user-facing changes?

Yes. The Arrow result type of AVG(decimal) may now be wider when the derived precision does not fit the input variant.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.79470% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.51%. Comparing base (dd77ffc) to head (5550865).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/functions-aggregate/src/average.rs	80.82%	3 Missing and 25 partials ⚠️
...egate-common/src/aggregate/avg_distinct/decimal.rs	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24371      +/-   ##
==========================================
- Coverage   81.51%   81.51%   -0.01%     
==========================================
  Files        1123     1123              
  Lines      405139   405235      +96     
  Branches   405139   405235      +96     
==========================================
+ Hits       330242   330308      +66     
- Misses      55568    55578      +10     
- Partials    19329    19349      +20     

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[kosiew]

@AdamGS, thanks for working on this. The changes look good overall. I left one small suggestion to add end-to-end coverage for the grouped AVG path.

[kosiew]

Could we add a small GROUP BY SQLLogicTest using Decimal32(7, 2) and assert that the result type is Decimal64(11, 6)? The unit tests already exercise the grouped state conversion, but an end-to-end grouped query would also verify that the planner selects the groups accumulator and that it produces the widened Arrow output type.

[AdamGS]

done!

[AdamGS]

@kosiew thank you for the quick review!

[kosiew]

🚀
@AdamGS
Thank you for your contribution.