refactor(proto): migrate scalar subquery serde

[Phoenix500526]

Which issue does this PR close?

• Closes Proto: migrate ScalarSubqueryExec (adds a subquery-results scope to the decode ctx) #23515.

Rationale for this change

ScalarSubqueryExpr nodes must share the results container populated by their enclosing ScalarSubqueryExec. A normal child decode loses that scope when serialization moves into the plan implementation.

Add a scoped child decode operation so the new plan hooks preserve the shared container while removing the central serialization path.

What changes are included in this PR?

Are these changes tested?

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 [  37.668s] (current)
     Parsing datafusion-physical-plan v54.1.0 (current)
      Parsed [   0.140s] (current)
    Building datafusion-physical-plan v54.1.0 (baseline)
       Built [  38.173s] (baseline)
     Parsing datafusion-physical-plan v54.1.0 (baseline)
      Parsed [   0.141s] (baseline)
    Checking datafusion-physical-plan v54.1.0 -> v54.1.0 (no change; assume patch)
     Checked [   0.650s] 223 checks: 222 pass, 1 fail, 0 warn, 30 skip

--- failure trait_method_added: pub trait method added ---

Description:
A non-sealed public trait added a new method without a default implementation, which breaks downstream implementations of the trait
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#trait-new-item-no-default
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/trait_method_added.ron

Failed in:
  trait method datafusion_physical_plan::proto::ExecutionPlanDecode::decode_plan_with_scalar_subquery_results in file /home/runner/work/datafusion/datafusion/datafusion/physical-plan/src/proto.rs:108

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [  78.025s] datafusion-physical-plan
    Building datafusion-proto v54.1.0 (current)
       Built [  59.774s] (current)
     Parsing datafusion-proto v54.1.0 (current)
      Parsed [   0.019s] (current)
    Building datafusion-proto v54.1.0 (baseline)
       Built [  59.824s] (baseline)
     Parsing datafusion-proto v54.1.0 (baseline)
      Parsed [   0.020s] (baseline)
    Checking datafusion-proto v54.1.0 -> v54.1.0 (no change; assume patch)
     Checked [   0.229s] 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_scalar_subquery_physical_plan in trait datafusion_proto::physical_plan::PhysicalPlanNodeExt in /home/runner/work/datafusion/datafusion/datafusion/proto/src/physical_plan/mod.rs:664
  method try_from_scalar_subquery_exec in trait datafusion_proto::physical_plan::PhysicalPlanNodeExt in /home/runner/work/datafusion/datafusion/datafusion/proto/src/physical_plan/mod.rs:664

     Summary semver requires new minor version: 0 major and 1 minor checks failed
    Finished [ 121.351s] datafusion-proto

[codecov-commenter]

Codecov Report

❌ Patch coverage is 67.56757% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.65%. Comparing base (e8a65f2) to head (6f12a16).

Files with missing lines	Patch %	Lines
datafusion/proto/src/physical_plan/mod.rs	38.46%	16 Missing ⚠️
datafusion/physical-plan/src/scalar_subquery.rs	80.00%	2 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23915      +/-   ##
==========================================
- Coverage   80.66%   80.65%   -0.02%     
==========================================
  Files        1095     1095              
  Lines      372294   372319      +25     
  Branches   372294   372319      +25     
==========================================
- Hits       300324   300296      -28     
- Misses      54055    54101      +46     
- Partials    17915    17922       +7     

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