refactor(proto): migrate UnnestExec serde

[Phoenix500526]

Which issue does this PR close?

• Closes Proto: migrate UnnestExec #23510.

Rationale for this change

The central protobuf dispatcher must know every built-in execution plan, which separates UnnestExec serialization from the type that owns it.

Move encoding and decoding to plan-local hooks while retaining the deprecated helpers as compatibility wrappers. Extend roundtrip coverage to include non-default unnest options.

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 v54.0.0 (current)
       Built [  30.685s] (current)
     Parsing datafusion-physical-plan v54.0.0 (current)
      Parsed [   0.117s] (current)
    Building datafusion-physical-plan v54.0.0 (baseline)
       Built [  29.118s] (baseline)
     Parsing datafusion-physical-plan v54.0.0 (baseline)
      Parsed [   0.117s] (baseline)
    Checking datafusion-physical-plan v54.0.0 -> v54.0.0 (no change; assume patch)
     Checked [   0.766s] 223 checks: 223 pass, 30 skip
     Summary no semver update required
    Finished [  62.989s] datafusion-physical-plan
    Building datafusion-proto v54.0.0 (current)
       Built [  47.877s] (current)
     Parsing datafusion-proto v54.0.0 (current)
      Parsed [   0.016s] (current)
    Building datafusion-proto v54.0.0 (baseline)
       Built [  48.430s] (baseline)
     Parsing datafusion-proto v54.0.0 (baseline)
      Parsed [   0.017s] (baseline)
    Checking datafusion-proto v54.0.0 -> v54.0.0 (no change; assume patch)
     Checked [   0.278s] 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/v0.48.0/src/lints/trait_method_marked_deprecated.ron

Failed in:
  method try_into_unnest_physical_plan in trait datafusion_proto::physical_plan::PhysicalPlanNodeExt in /home/runner/work/datafusion/datafusion/datafusion/proto/src/physical_plan/mod.rs:719
  method try_from_unnest_exec in trait datafusion_proto::physical_plan::PhysicalPlanNodeExt in /home/runner/work/datafusion/datafusion/datafusion/proto/src/physical_plan/mod.rs:719

     Summary semver requires new minor version: 0 major and 1 minor checks failed
    Finished [  98.057s] datafusion-proto

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.06422% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.68%. Comparing base (9a50c67) to head (04a2a38).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/proto/src/physical_plan/mod.rs	6.66%	14 Missing ⚠️
datafusion/physical-plan/src/unnest.rs	88.29%	5 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23739      +/-   ##
==========================================
- Coverage   80.69%   80.68%   -0.02%     
==========================================
  Files        1089     1089              
  Lines      368424   368486      +62     
  Branches   368424   368486      +62     
==========================================
+ Hits       297307   297317      +10     
- Misses      53406    53451      +45     
- Partials    17711    17718       +7     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.