fix(substrait): consume chained window functions whose default names collide

[ShayanGho]

Which issue does this PR close?

• Closes Substrait consumer: DuplicateUnqualifiedField when chaining two Window relations with a carried window column #23007.

Rationale for this change

A logical plan that contains two window aggregates with the same default schema name, distinguished only by aliases, executes fine in DataFusion but cannot be consumed back after a Substrait round trip. The consumer fails with:

SchemaError(DuplicateUnqualifiedField { name: "avg(data.b) PARTITION BY [data.a] ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW" })

DataFusion's Substrait producer uses positional projections and does not preserve intermediate window aliases: a ProjectRel carries expressions and an output mapping. The aliases that kept the two columns apart in DataFusion (AS avg1, AS avg2) are dropped on the wire. When the consumer rebuilds the outer projection, the inherited window column and the new window expression both come back under the same default name, and Window::try_new rejects the plan.

The SQL from the issue reproduces it, and so does a minimal plan built with LogicalPlanBuilder that stacks two row_number() windows.

What changes are included in this PR?

Two small changes, one per crate:

• datafusion-expr: group_window_expr_by_sort_keys (used by LogicalPlanBuilder::window_plan) now looks through a single Expr::Alias to derive the sort key and keeps the aliased expression in its group, so window_plan can build a Window node whose output field carries an alias. LogicalPlanBuilder::window, filter pushdown, and the physical planner already handle aliased window expressions, so this makes window_plan consistent with them. Nested aliases are still rejected, matching what filter pushdown tolerates.
• datafusion-substrait consumer: from_project_rel reserves the input schema's field names in a NameTracker (new reserve_schema helper), aliases any new window expression whose default name would collide, and rewrites the projection to reference the alias while preserving the projection's own output names via NamePreserver. Plans without a collision are built exactly as before.

What is the testing strategy for this PR?

• datafusion-expr: a unit test (test_group_window_expr_by_sort_keys_aliased_window_expr) asserting that an aliased window expression is grouped by the inner function's sort key and kept aliased. It failed on main with Impossibly got non-window expr.
• datafusion-substrait: two round-trip tests in roundtrip_logical_plan.rs, stacked_windows_with_same_default_name_via_builder (minimal LogicalPlanBuilder form) and chained_windows_with_same_default_name (the SQL from the issue). Both failed before this change with the DuplicateUnqualifiedField error above. They assert Arrow schema equality (names, types, nullability) and execute the consumed plan. They do not compare plan text, because the consumer must synthesize an alias for the colliding window, so the text legitimately differs. They compare Arrow schemas rather than full DFSchemas because qualifiers and inferred functional dependencies can change during reconstruction: DataFusion's Substrait producer does not preserve subquery aliases, and the rebuilt Window nodes infer dependencies the original plan did not record.
• The full datafusion-substrait test run passes unchanged (273 passed, 6 ignored across the lib, integration, and doc tests), confirming the non-colliding path is unaffected.

This change was developed with AI assistance. I have reviewed and understand every line and stand behind it.

Are there any user-facing changes?

No API changes. Plans consumed from Substrait that previously failed with DuplicateUnqualifiedField now consume successfully. Synthesized aliases of the form <default name>__temp__N may appear in the decoded logical plan; final output column names are preserved. Plans without such a collision are unchanged.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.56627% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.91%. Comparing base (7e5f40a) to head (4dbab95).

Files with missing lines	Patch %	Lines
...trait/src/logical_plan/consumer/rel/project_rel.rs	90.47%	0 Missing and 4 partials ⚠️
datafusion/expr/src/utils.rs	91.66%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25181      +/-   ##
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- Coverage   81.91%   81.91%   -0.01%     
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  Files        1132     1132              
  Lines      421117   421189      +72     
  Branches   421117   421189      +72     
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+ Hits       344961   345016      +55     
- Misses      55767    55776       +9     
- Partials    20389    20397       +8     

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