fix: do not prune ORDER BY keys using nullable UNIQUE constraints

[buraksenn]

Which issue does this PR close?

• Closes Incorrect results: ORDER BY drops a sort key based on a nullable UNIQUE constraint #23818.

Rationale for this change

Please check the issue for details, but the main idea is that a nullable UNIQUE column permits multiple NULL rows, so it does not determine later sort keys across them and they must not be pruned.

What changes are included in this PR?

FunctionalDependencies tracks a NullEquality per dependency. Sort-key pruning now requires the determinant to treat NULLs as equal (GROUP BY key), be non-nullable, or have NOT NULL sources.

Are these changes tested?

Yes, regression tests added in functional_dependencies.slt.

Are there any user-facing changes?

No API changes; incorrect results are fixed.

[buraksenn]

this can be added in FunctionalDependence but since that is pub as well I did not want to change this. I can add this field into that struct if that makes more sense

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.66%. Comparing base (36c417b) to head (3f37107).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##             main   #23918     +/-   ##
=========================================
  Coverage   80.65%   80.66%             
=========================================
  Files        1092     1095      +3     
  Lines      371140   372344   +1204     
  Branches   371140   372344   +1204     
=========================================
+ Hits       299353   300361   +1008     
- Misses      53933    54063    +130     
- Partials    17854    17920     +66     

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