refactor: share join input unparsing

[Xuanwo]

Which issue does this PR close?

• Part of ASOF join support / Specialize Range Joins #318.
• Umbrella PR: feat: support ASOF joins #23738.
• Follow-up to feat: support ASOF JOIN SQL #23830.

Rationale for this change

Regular and ASOF join unparsing separately handled projected inputs, nested right joins, and output projection reconstruction. Keeping those branches separate makes fixes easy to apply to one join kind but miss the other.

GitHub shows the cumulative stack because fork branches cannot be selected as upstream PR bases. The isolated diff is:
Xuanwo/datafusion@xuanwo/asof-sql...xuanwo/asof-unparser-refactor

What changes are included in this PR?

• Route regular and ASOF join inputs through one private preparation path.
• Keep left-side join chaining, right-side nesting, and ASOF derived-input boundaries explicit through JoinInputMode.
• Preserve existing SQL output and projection behavior.

Are these changes tested?

Yes:

• cargo fmt --all
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test -p datafusion-sql --test sql_integration
• Extended workspace tests from the contributor guide

Are there any user-facing changes?

No. This is a behavior-preserving unparser refactor.

[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 v55.0.0 (current)
       Built [  57.712s] (current)
     Parsing datafusion v55.0.0 (current)
      Parsed [   0.033s] (current)
    Building datafusion v55.0.0 (baseline)
       Built [  57.068s] (baseline)
     Parsing datafusion v55.0.0 (baseline)
      Parsed [   0.033s] (baseline)
    Checking datafusion v55.0.0 -> v55.0.0 (no change; assume patch)
     Checked [   0.746s] 223 checks: 223 pass, 31 skip
     Summary no semver update required
    Finished [ 117.425s] datafusion
    Building datafusion-expr v55.0.0 (current)
       Built [  28.544s] (current)
     Parsing datafusion-expr v55.0.0 (current)
      Parsed [   0.075s] (current)
    Building datafusion-expr v55.0.0 (baseline)
       Built [  28.625s] (baseline)
     Parsing datafusion-expr v55.0.0 (baseline)
      Parsed [   0.076s] (baseline)
    Checking datafusion-expr v55.0.0 -> v55.0.0 (no change; assume patch)
     Checked [   1.442s] 223 checks: 222 pass, 1 fail, 0 warn, 31 skip

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/inherent_method_missing.ron

Failed in:
  LogicalPlanBuilder::asof_join, previously in file /home/runner/work/datafusion/datafusion/target/semver-checks/git-apache_main/13b4144fa032bf81b7ddcf1c2dc2dc618eedbec2/datafusion/expr/src/logical_plan/builder.rs:1012
  LogicalPlanBuilder::asof_join, previously in file /home/runner/work/datafusion/datafusion/target/semver-checks/git-apache_main/13b4144fa032bf81b7ddcf1c2dc2dc618eedbec2/datafusion/expr/src/logical_plan/builder.rs:1012
  LogicalPlanBuilder::asof_join, previously in file /home/runner/work/datafusion/datafusion/target/semver-checks/git-apache_main/13b4144fa032bf81b7ddcf1c2dc2dc618eedbec2/datafusion/expr/src/logical_plan/builder.rs:1012
  LogicalPlanBuilder::asof_join, previously in file /home/runner/work/datafusion/datafusion/target/semver-checks/git-apache_main/13b4144fa032bf81b7ddcf1c2dc2dc618eedbec2/datafusion/expr/src/logical_plan/builder.rs:1012

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [  59.851s] datafusion-expr
    Building datafusion-sql v55.0.0 (current)
       Built [  42.632s] (current)
     Parsing datafusion-sql v55.0.0 (current)
      Parsed [   0.030s] (current)
    Building datafusion-sql v55.0.0 (baseline)
       Built [  42.670s] (baseline)
     Parsing datafusion-sql v55.0.0 (baseline)
      Parsed [   0.032s] (baseline)
    Checking datafusion-sql v55.0.0 -> v55.0.0 (no change; assume patch)
     Checked [   0.286s] 223 checks: 223 pass, 31 skip
     Summary no semver update required
    Finished [  86.859s] datafusion-sql
    Building datafusion-sqllogictest v55.0.0 (current)
       Built [  97.055s] (current)
     Parsing datafusion-sqllogictest v55.0.0 (current)
      Parsed [   0.021s] (current)
    Building datafusion-sqllogictest v55.0.0 (baseline)
       Built [  97.635s] (baseline)
     Parsing datafusion-sqllogictest v55.0.0 (baseline)
      Parsed [   0.022s] (baseline)
    Checking datafusion-sqllogictest v55.0.0 -> v55.0.0 (no change; assume patch)
     Checked [   0.106s] 223 checks: 223 pass, 31 skip
     Summary no semver update required
    Finished [ 197.580s] datafusion-sqllogictest
    Building datafusion-substrait v55.0.0 (current)
       Built [ 297.204s] (current)
     Parsing datafusion-substrait v55.0.0 (current)
      Parsed [   0.016s] (current)
    Building datafusion-substrait v55.0.0 (baseline)
       Built [ 297.973s] (baseline)
     Parsing datafusion-substrait v55.0.0 (baseline)
      Parsed [   0.019s] (baseline)
    Checking datafusion-substrait v55.0.0 -> v55.0.0 (no change; assume patch)
     Checked [   0.251s] 223 checks: 223 pass, 31 skip
     Summary no semver update required
    Finished [ 597.236s] datafusion-substrait

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.37092% with 83 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.68%. Comparing base (9fc7a4d) to head (c2bd4a2).
⚠️ Report is 21 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/sql/src/unparser/plan.rs	67.03%	51 Missing and 9 partials ⚠️
datafusion/sql/src/relation/join.rs	77.35%	5 Missing and 7 partials ⚠️
datafusion/expr/src/logical_plan/builder.rs	87.77%	3 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24965      +/-   ##
==========================================
+ Coverage   81.57%   81.68%   +0.10%     
==========================================
  Files        1123     1124       +1     
  Lines      410789   413490    +2701     
  Branches   410789   413490    +2701     
==========================================
+ Hits       335120   337765    +2645     
+ Misses      55913    55829      -84     
- Partials    19756    19896     +140     

☔ 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.