refactor(proto): migrate aggregate exec serde

[Phoenix500526]

Which issue does this PR close?

• Closes Proto: migrate AggregateExec (function-codec path) #23512.

Rationale for this change

Aggregate protobuf conversion lived in the central proto crate, which prevented AggregateExec from owning its function-codec serialization.

Move encoding and decoding into AggregateExec and keep the deprecated helpers as delegates so existing callers retain wire-compatible behavior.

What changes are included in this PR?

Are these changes tested?

Yes

Are there any user-facing changes?

[github-actions]

Thank you for opening this pull request!

Reviewer note: cargo-semver-checks reported the current version number is not SemVer-compatible with the changes in this pull request (compared against the base branch).

Details

     Cloning apache/main
    Building datafusion-physical-plan v54.1.0 (current)
       Built [  39.424s] (current)
     Parsing datafusion-physical-plan v54.1.0 (current)
      Parsed [   0.132s] (current)
    Building datafusion-physical-plan v54.1.0 (baseline)
       Built [  34.979s] (baseline)
     Parsing datafusion-physical-plan v54.1.0 (baseline)
      Parsed [   0.134s] (baseline)
    Checking datafusion-physical-plan v54.1.0 -> v54.1.0 (no change; assume patch)
     Checked [   0.703s] 223 checks: 223 pass, 30 skip
     Summary no semver update required
    Finished [  77.328s] datafusion-physical-plan
    Building datafusion-proto v54.1.0 (current)
       Built [  56.458s] (current)
     Parsing datafusion-proto v54.1.0 (current)
      Parsed [   0.018s] (current)
    Building datafusion-proto v54.1.0 (baseline)
       Built [  56.513s] (baseline)
     Parsing datafusion-proto v54.1.0 (baseline)
      Parsed [   0.019s] (baseline)
    Checking datafusion-proto v54.1.0 -> v54.1.0 (no change; assume patch)
     Checked [   0.247s] 223 checks: 222 pass, 1 fail, 0 warn, 30 skip

--- failure trait_method_marked_deprecated: trait method #[deprecated] added ---

Description:
A trait method is now #[deprecated]. Downstream crates will get a compiler warning when using this method.
        ref: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-deprecated-attribute
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/trait_method_marked_deprecated.ron

Failed in:
  method try_into_aggregate_physical_plan in trait datafusion_proto::physical_plan::PhysicalPlanNodeExt in /home/runner/work/datafusion/datafusion/datafusion/proto/src/physical_plan/mod.rs:680
  method try_from_aggregate_exec in trait datafusion_proto::physical_plan::PhysicalPlanNodeExt in /home/runner/work/datafusion/datafusion/datafusion/proto/src/physical_plan/mod.rs:680

     Summary semver requires new minor version: 0 major and 1 minor checks failed
    Finished [ 114.630s] datafusion-proto

[codecov-commenter]

Codecov Report

❌ Patch coverage is 76.14035% with 68 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.69%. Comparing base (050046a) to head (df27c52).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/physical-plan/src/aggregates/mod.rs	80.00%	23 Missing and 31 partials ⚠️
datafusion/proto/src/physical_plan/mod.rs	6.66%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23779      +/-   ##
==========================================
- Coverage   80.71%   80.69%   -0.02%     
==========================================
  Files        1089     1089              
  Lines      368750   368776      +26     
  Branches   368750   368776      +26     
==========================================
- Hits       297631   297582      -49     
- Misses      53375    53445      +70     
- Partials    17744    17749       +5     

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