feat: migrate EmptyExec and PlaceholderRowExec to ExecutionPlan proto hooks

[847850277]

Which issue does this PR close?

• Closes Proto: migrate leaf plans EmptyExec + PlaceholderRowExec #23501 .

Rationale for this change

Migrates the EmptyExec and PlaceholderRowExec leaf plans while preserving the existing protobuf wire format.

What changes are included in this PR?

• Implement ExecutionPlan::try_to_proto for EmptyExec and PlaceholderRowExec.
• Add plan-specific try_from_proto implementations.
• Route protobuf encoding and decoding through the new hooks.
• Keep the deprecated helper methods as compatibility shims.

Are these changes tested?

yes

Are there any user-facing changes?

Hi @andygrove , would you be willing to review this PR when you have time? Thanks!

[Copilot]

Pull request overview

This PR migrates the EmptyExec and PlaceholderRowExec physical leaf plans to the newer protobuf “plan self-serialization” hooks (ExecutionPlan::try_to_proto / X::try_from_proto) while preserving the existing wire format. This continues the ongoing effort to move protobuf encoding/decoding responsibility closer to each plan type and reduce the central downcast/match chains in datafusion-proto.

Changes:

• Implement ExecutionPlan::try_to_proto for EmptyExec and PlaceholderRowExec in datafusion-physical-plan.
• Add plan-specific try_from_proto constructors for EmptyExec and PlaceholderRowExec.
• Route protobuf decoding through these new hooks and remove the old central encoding downcast arms, keeping deprecated helper methods as compatibility shims.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
datafusion/proto/src/physical_plan/mod.rs	Switch decode match arms to X::try_from_proto, remove central encoding downcast arms for these leaf plans, and keep deprecated helpers as shims delegating to the new hooks.
datafusion/physical-plan/src/placeholder_row.rs	Add try_to_proto and PlaceholderRowExec::try_from_proto implementations under feature = "proto".
datafusion/physical-plan/src/empty.rs	Add try_to_proto and EmptyExec::try_from_proto implementations under feature = "proto".

---

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[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 [  36.152s] (current)
     Parsing datafusion-physical-plan v54.1.0 (current)
      Parsed [   0.148s] (current)
    Building datafusion-physical-plan v54.1.0 (baseline)
       Built [  35.601s] (baseline)
     Parsing datafusion-physical-plan v54.1.0 (baseline)
      Parsed [   0.152s] (baseline)
    Checking datafusion-physical-plan v54.1.0 -> v54.1.0 (no change; assume patch)
     Checked [   0.923s] 223 checks: 223 pass, 30 skip
     Summary no semver update required
    Finished [  74.440s] datafusion-physical-plan
    Building datafusion-proto v54.1.0 (current)
       Built [  58.751s] (current)
     Parsing datafusion-proto v54.1.0 (current)
      Parsed [   0.020s] (current)
    Building datafusion-proto v54.1.0 (baseline)
       Built [  58.634s] (baseline)
     Parsing datafusion-proto v54.1.0 (baseline)
      Parsed [   0.021s] (baseline)
    Checking datafusion-proto v54.1.0 -> v54.1.0 (no change; assume patch)
     Checked [   0.340s] 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_empty_physical_plan in trait datafusion_proto::physical_plan::PhysicalPlanNodeExt in /home/runner/work/datafusion/datafusion/datafusion/proto/src/physical_plan/mod.rs:722
  method try_into_placeholder_row_physical_plan in trait datafusion_proto::physical_plan::PhysicalPlanNodeExt in /home/runner/work/datafusion/datafusion/datafusion/proto/src/physical_plan/mod.rs:722
  method try_from_empty_exec in trait datafusion_proto::physical_plan::PhysicalPlanNodeExt in /home/runner/work/datafusion/datafusion/datafusion/proto/src/physical_plan/mod.rs:722
  method try_from_placeholder_row_exec in trait datafusion_proto::physical_plan::PhysicalPlanNodeExt in /home/runner/work/datafusion/datafusion/datafusion/proto/src/physical_plan/mod.rs:722

     Summary semver requires new minor version: 0 major and 1 minor checks failed
    Finished [ 119.514s] datafusion-proto

[codecov-commenter]

Codecov Report

❌ Patch coverage is 51.37615% with 53 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.69%. Comparing base (5de7f1d) to head (967c518).

Files with missing lines	Patch %	Lines
datafusion/proto/src/physical_plan/mod.rs	4.44%	43 Missing ⚠️
datafusion/physical-plan/src/empty.rs	83.87%	2 Missing and 3 partials ⚠️
datafusion/physical-plan/src/placeholder_row.rs	84.84%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23784      +/-   ##
==========================================
- Coverage   80.71%   80.69%   -0.02%     
==========================================
  Files        1089     1089              
  Lines      368760   368828      +68     
  Branches   368760   368828      +68     
==========================================
- Hits       297647   297631      -16     
- Misses      53372    53446      +74     
- Partials    17741    17751      +10     

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