refactor: moving WindowTopN before EnsureRequirements

[saadtajwar]

Which issue does this PR close?

• Closes Simplify WindowTopN by moving it before EnforceSorting #21594

Rationale for this change

The WindowTopN physical optimizer rule currently runs after EnsureRequirements, which means it must pattern-match through SortExec nodes that EnsureRequirements inserts:

FilterExec(rn <= K)                                                                                                                                                               
  [optional ProjectionExec]                                                                                                                                                       
    BoundedWindowAggExec(ROW_NUMBER PARTITION BY ... ORDER BY ...)
      SortExec(partition_keys, order_keys)   ← inserted by EnforceSorting

By moving WindowTopN to run before EnsureRequirements, we can simplify the logic to avoid pattern-matching through SortExec node and instead use the ORDER BY and PARTITION BY on the BoundedWindowAggExec

What changes are included in this PR?

Moved WindowTopN to run before EnsureRequirements, and updated its logic to not expect a SortExec and instead use the ORDER BY and PARTITION BY on the BoundedWindowAggExec

Are these changes tested?

Yes

Are there any user-facing changes?

These are just optimizer changes (order of optimizations and internal logic of WindowTopN), no API changes

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.98%. Comparing base (d5bd10d) to head (7a5bccd).

Files with missing lines	Patch %	Lines
datafusion/physical-optimizer/src/window_topn.rs	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24191      +/-   ##
==========================================
- Coverage   80.98%   80.98%   -0.01%     
==========================================
  Files        1106     1106              
  Lines      383495   383498       +3     
  Branches   383495   383498       +3     
==========================================
  Hits       310590   310590              
+ Misses      54572    54571       -1     
- Partials    18333    18337       +4     

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[saadtajwar]

@SubhamSinghal & @2010YOUY01 - ready for review if you two have the time! Thanks so much in advance!

[saadtajwar]

Thank you @jayzhan211 for the review! Please let me know if there's anything else needed on my end to get this in :)