Allow Spark partial / Comet final for compatible aggregates

[Shekharrajak]

Which issue does this PR close?

Closes #2894

Rationale for this change

Comet currently falls back to Spark for ALL final hash aggregates when there's no Comet partial aggregate in the child plan. This is overly conservative because some aggregates have compatible intermediate buffer formats between Spark and Comet.
For example, MIN, MAX, COUNT, and bitwise aggregates (BIT_AND, BIT_OR, BIT_XOR) have simple intermediate buffers (single value) that are compatible between Spark and Comet. These can safely run with "Spark partial / Comet final" execution.
Other aggregates like SUM, AVG, VARIANCE, etc. have known incompatibilities (e.g., decimal overflow handling differences, complex intermediate buffers) and should continue to fall back when there's no Comet partial aggregate.

What changes are included in this PR?

Added supportsSparkPartialCometFinal method to CometAggregateExpressionSerde trait - Default is false

Added helper function - aggSupportsMixedExecution() in QueryPlanSerde

How are these changes tested?

"CometExecRule should not allow Spark partial and Comet final for unsafe aggregates" - Verifies SUM still falls back to Spark

"CometExecRule should allow Spark partial and Comet final for safe aggregates" - Verifies MIN/MAX/COUNT can use Comet final with Spark partial

[codecov-commenter]

Codecov Report

❌ Patch coverage is 53.84615% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.58%. Comparing base (f09f8af) to head (51869b1).
⚠️ Report is 803 commits behind head on main.

Files with missing lines	Patch %	Lines
...main/scala/org/apache/comet/serde/aggregates.scala	50.00%	3 Missing ⚠️
.../scala/org/apache/comet/serde/QueryPlanSerde.scala	33.33%	1 Missing and 1 partial ⚠️
...n/scala/org/apache/spark/sql/comet/operators.scala	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2994      +/-   ##
============================================
- Coverage     56.12%   54.58%   -1.54%     
- Complexity      976     1256     +280     
============================================
  Files           119      167      +48     
  Lines         11743    15505    +3762     
  Branches       2251     2571     +320     
============================================
+ Hits           6591     8464    +1873     
- Misses         4012     5822    +1810     
- Partials       1140     1219      +79     

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