refactor(proto): migrate AsyncFuncExec to self-serializing proto

[mattp5657]

Closes #23514. Part of #23494.

Migrate AsyncFuncExec proto encode/decode into the plan itself via try_to_proto / try_from_proto, removing its central-arm handling in physical_plan/mod.rs.

Rationale

datafusion-proto currently downcasts AsyncFuncExec in the central encode match and rebuilds it inline on decode. #23495 introduced self-serializing hooks so each plan owns its own wire format. This moves AsyncFuncExec onto that pattern.

Changes

• Added try_to_proto / AsyncFuncExec::try_from_proto, wired into the decode dispatch
• Removed the central encode downcast branch
• Old helper methods kept as #[deprecated] stubs, per existing convention
• Wire format unchanged

Testing

Existing roundtrip_async_func_exec integration test now exercises the new hooks (old path deleted, so it's the only path left). cargo fmt + cargo clippy --all-targets --features proto -- -D warnings clean.

User-facing changes

No

[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 [  36.879s] (current)
     Parsing datafusion-physical-plan v54.1.0 (current)
      Parsed [   0.148s] (current)
    Building datafusion-physical-plan v54.1.0 (baseline)
       Built [  35.803s] (baseline)
     Parsing datafusion-physical-plan v54.1.0 (baseline)
      Parsed [   0.149s] (baseline)
    Checking datafusion-physical-plan v54.1.0 -> v54.1.0 (no change; assume patch)
     Checked [   0.938s] 223 checks: 223 pass, 30 skip
     Summary no semver update required
    Finished [  75.409s] datafusion-physical-plan
    Building datafusion-proto v54.1.0 (current)
       Built [  59.376s] (current)
     Parsing datafusion-proto v54.1.0 (current)
      Parsed [   0.021s] (current)
    Building datafusion-proto v54.1.0 (baseline)
       Built [  64.887s] (baseline)
     Parsing datafusion-proto v54.1.0 (baseline)
      Parsed [   0.023s] (baseline)
    Checking datafusion-proto v54.1.0 -> v54.1.0 (no change; assume patch)
     Checked [   0.330s] 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_async_func_physical_plan in trait datafusion_proto::physical_plan::PhysicalPlanNodeExt in /home/runner/work/datafusion/datafusion/datafusion/proto/src/physical_plan/mod.rs:721
  method try_from_async_func_exec in trait datafusion_proto::physical_plan::PhysicalPlanNodeExt in /home/runner/work/datafusion/datafusion/datafusion/proto/src/physical_plan/mod.rs:721

     Summary semver requires new minor version: 0 major and 1 minor checks failed
    Finished [ 126.070s] datafusion-proto

[codecov-commenter]

Codecov Report

❌ Patch coverage is 65.27778% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.70%. Comparing base (c118002) to head (63b1850).

Files with missing lines	Patch %	Lines
datafusion/proto/src/physical_plan/mod.rs	5.26%	18 Missing ⚠️
datafusion/physical-plan/src/async_func.rs	86.79%	2 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23825      +/-   ##
==========================================
- Coverage   80.72%   80.70%   -0.02%     
==========================================
  Files        1089     1089              
  Lines      368911   368932      +21     
  Branches   368911   368932      +21     
==========================================
- Hits       297785   297762      -23     
- Misses      53374    53409      +35     
- Partials    17752    17761       +9     

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