refactor(proto): migrate sort merge join serde

[Phoenix500526]

Which issue does this PR close?

• Closes Proto: migrate SortMergeJoinExec #23508 .

Rationale for this change

SortMergeJoinExec serialization currently depends on centralized
encoding and decoding logic in datafusion-proto. This keeps protobuf
ownership separate from the execution plan and contributes to the
central type-dispatch chain tracked by #23494.

Moving serialization into SortMergeJoinExec keeps the implementation
with the plan that owns the relevant state and aligns it with the new
per-plan serialization model.

What changes are included in this PR?

• Add ExecutionPlan::try_to_proto for SortMergeJoinExec.
• Add SortMergeJoinExec::try_from_proto.
• Preserve serialization of:
  • Both child plans.
  • Join-key expressions.
  • Join type and null-equality behavior.
  • Optional join filters and column origins.
  • Per-key sort options.
• Convert JoinType, JoinSide, and NullEquality through exhaustive
  by-name matches.
• Route decoding through the plan-local implementation and remove the
  centralized encoding downcast.
• Retain the old public conversion helpers as deprecated compatibility
  APIs.
• Preserve the existing protobuf wire representation.
• Expand roundtrip coverage for every join type, both null-equality
  modes, filters, column origins, and non-default sort options.

Are these changes tested?

Yes

Are there any user-facing changes?

There are no runtime behavior or protobuf wire-format changes.

The old PhysicalPlanNodeExt conversion helpers remain available but
are deprecated in favor of the plan-local serialization hooks.

[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 [  38.755s] (current)
     Parsing datafusion-physical-plan v54.0.0 (current)
      Parsed [   0.143s] (current)
    Building datafusion-physical-plan v54.0.0 (baseline)
       Built [  37.691s] (baseline)
     Parsing datafusion-physical-plan v54.0.0 (baseline)
      Parsed [   0.144s] (baseline)
    Checking datafusion-physical-plan v54.0.0 -> v54.0.0 (no change; assume patch)
     Checked [   0.679s] 223 checks: 223 pass, 30 skip
     Summary no semver update required
    Finished [  78.975s] datafusion-physical-plan
    Building datafusion-proto v54.0.0 (current)
       Built [  61.841s] (current)
     Parsing datafusion-proto v54.0.0 (current)
      Parsed [   0.018s] (current)
    Building datafusion-proto v54.0.0 (baseline)
       Built [  61.855s] (baseline)
     Parsing datafusion-proto v54.0.0 (baseline)
      Parsed [   0.020s] (baseline)
    Checking datafusion-proto v54.0.0 -> v54.0.0 (no change; assume patch)
     Checked [   0.249s] 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_sort_join in trait datafusion_proto::physical_plan::PhysicalPlanNodeExt in /home/runner/work/datafusion/datafusion/datafusion/proto/src/physical_plan/mod.rs:717
  method try_from_sort_merge_join_exec in trait datafusion_proto::physical_plan::PhysicalPlanNodeExt in /home/runner/work/datafusion/datafusion/datafusion/proto/src/physical_plan/mod.rs:717

     Summary semver requires new minor version: 0 major and 1 minor checks failed
    Finished [ 125.679s] datafusion-proto

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.58974% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.65%. Comparing base (a8d1af6) to head (a535900).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...on/physical-plan/src/joins/sort_merge_join/exec.rs	83.50%	16 Missing and 16 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23712      +/-   ##
==========================================
- Coverage   80.69%   80.65%   -0.04%     
==========================================
  Files        1088     1088              
  Lines      367741   367928     +187     
  Branches   367741   367928     +187     
==========================================
+ Hits       296761   296768       +7     
- Misses      53272    53448     +176     
- Partials    17708    17712       +4     

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[adriangb]

Can we have these deprecated methods delegate to the new methods so we avoid code duplication?