fix: preserve ProjectionExec metadata during serialization

[goutamadwant]

Which issue does this PR close?

• Closes Physical-plan serialization drops ProjectionExec metadata #24695.

Rationale for this change

Physical-plan serialization loses projection field and schema metadata when it differs from metadata derived from the input and expressions.

What changes are included in this PR?

• Add an optional schema to ProjectionExecNode, emitted only when the projection overrides metadata.
• Restore metadata through try_new_with_schema_metadata, keeping field names, types, and nullability derived from the expressions.
• Preserve decoding of older payloads without the optional field.

What is the testing strategy for this PR?

• roundtrip_projection_metadata reproduces the loss before the fix and passes afterward.
• Added binary and JSON coverage for overrides, explicit clearing, extension metadata, and older payloads. decode_projection_schema_only_replaces_metadata checks expression-derived field properties and invalid field counts.
• The required extended workspace test command passes, including 2,014 physical-plan tests, 268 protobuf integration tests, and 511 SQL logic-test files.
• All-target, all-feature Clippy passes for the three affected crates; the contributor guide's documented lint steps also pass.
• Full-workspace Clippy with all features enabled hits the existing PostgreSQL decimal-formatting lint in Full workspace clippy fails in PostgreSQL decimal formatting #24974; the affected source is unchanged here.

Are there any user-facing changes?

Projection metadata survives physical-plan serialization. The generated ProjectionExecNode Rust struct gains an optional schema field. New readers continue to accept older payloads. This targets main, not a 55.1 backport.

[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 v55.0.0 (current)
       Built [  40.801s] (current)
     Parsing datafusion-physical-plan v55.0.0 (current)
      Parsed [   0.157s] (current)
    Building datafusion-physical-plan v55.0.0 (baseline)
       Built [  39.944s] (baseline)
     Parsing datafusion-physical-plan v55.0.0 (baseline)
      Parsed [   0.157s] (baseline)
    Checking datafusion-physical-plan v55.0.0 -> v55.0.0 (no change; assume patch)
     Checked [   0.693s] 223 checks: 223 pass, 31 skip
     Summary no semver update required
    Finished [  83.677s] datafusion-physical-plan
    Building datafusion-proto v55.0.0 (current)
       Built [  55.621s] (current)
     Parsing datafusion-proto v55.0.0 (current)
      Parsed [   0.018s] (current)
    Building datafusion-proto v55.0.0 (baseline)
       Built [  54.730s] (baseline)
     Parsing datafusion-proto v55.0.0 (baseline)
      Parsed [   0.019s] (baseline)
    Checking datafusion-proto v55.0.0 -> v55.0.0 (no change; assume patch)
     Checked [   0.115s] 223 checks: 223 pass, 31 skip
     Summary no semver update required
    Finished [ 111.737s] datafusion-proto
    Building datafusion-proto-models v55.0.0 (current)
       Built [  25.712s] (current)
     Parsing datafusion-proto-models v55.0.0 (current)
      Parsed [   0.131s] (current)
    Building datafusion-proto-models v55.0.0 (baseline)
       Built [  25.619s] (baseline)
     Parsing datafusion-proto-models v55.0.0 (baseline)
      Parsed [   0.134s] (baseline)
    Checking datafusion-proto-models v55.0.0 -> v55.0.0 (no change; assume patch)
     Checked [   1.711s] 223 checks: 222 pass, 1 fail, 0 warn, 31 skip

--- failure constructible_struct_adds_field: struct exhaustively constructible through public API adds field ---

Description:
A pub struct that could be exhaustively constructed with a literal using only public API has a new pub field, breaking existing exhaustive literals.
        ref: https://doc.rust-lang.org/reference/expressions/struct-expr.html
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/constructible_struct_adds_field.ron

Failed in:
  field ProjectionExecNode.schema in /home/runner/work/datafusion/datafusion/datafusion/proto-models/src/generated/prost.rs:2207
  field ProjectionExecNode.schema in /home/runner/work/datafusion/datafusion/datafusion/proto-models/src/generated/prost.rs:2207

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [  54.657s] datafusion-proto-models

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.77778% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.75%. Comparing base (e1ca94f) to head (d58dcb7).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/physical-plan/src/projection.rs	78.57%	0 Missing and 3 partials ⚠️
datafusion/proto-models/src/generated/pbjson.rs	76.92%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25009      +/-   ##
==========================================
+ Coverage   81.69%   81.75%   +0.05%     
==========================================
  Files        1127     1127              
  Lines      415471   415680     +209     
  Branches   415471   415680     +209     
==========================================
+ Hits       339424   339844     +420     
+ Misses      56108    55850     -258     
- Partials    19939    19986      +47     

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