fix(substrait): plan nested projected window expressions

[bvolpato]

Which issue does this PR close?

• Closes Substrait consumer does not plan nested window expressions in projections #22629.

Rationale for this change

The Substrait ProjectRel consumer only added a WindowAggr relation when the root projected expression was a WindowFunction. A scalar expression wrapping a valid window function therefore left the window directly inside Projection, which cannot be physically planned.

Minimal reproducer represented by the added Substrait fixture:

SELECT 1 + count(*) OVER () FROM DATA;

Before this patch, the regression test produced this plan difference:

 Projection: Int64(1) + count(Int64(1)) ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING AS EXPR$0
-  WindowAggr: windowExpr=[[count(Int64(1)) ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING]]
-    TableScan: DATA
+  TableScan: DATA

This is the same class of failure that reaches physical planning as an unsupported nested WindowFunction expression.

What changes are included in this PR?

• Use existing find_window_exprs(...) recursion while consuming Substrait ProjectRel expressions, retaining current HashSet deduplication across projections.
• Add a minimal Substrait JSON fixture with a window function nested inside an arithmetic scalar expression.
• Add a logical-plan snapshot plus execution regression, proving WindowAggr is inserted and physically executable.

Are these changes tested?

Pre-fix evidence:

• cargo test -p datafusion-substrait --test substrait_integration nested_window_function_in_expression -- --nocapture failed because the consumed plan omitted expected WindowAggr and put Projection directly above TableScan.

Final validation on Apache main commit d8c458828:

• cargo fmt --all -- --check
• cargo test -p datafusion-substrait (49 unit passed, 200 integration passed, 3 doctests passed; 6 existing ignored)
• cargo check --all-targets -p datafusion-substrait
• cargo check --no-default-features -p datafusion-substrait
• cargo check --no-default-features -p datafusion-substrait --features=physical
• cargo check --no-default-features -p datafusion-substrait --features=protoc
• cargo clippy --all-targets --all-features -- -D warnings
• ./dev/rust_lint.sh

Are there any user-facing changes?

Substrait producers may now send projected expressions that contain nested window functions; DataFusion consumes them into executable logical plans instead of leaving unsupported window expressions in projections.