preserve visitor rewrites in window function SQL expressions

[koenbeuk]

WindowFunctionSqlExpression.VisitChildren served two masters: rendering SQL for
QuerySqlGenerator and answering EF's rewrite passes — and it discarded the
latter's results, so alias remapping and pushdown silently lost their rewrites.
Now self-renders only for QuerySqlGenerator and rebuilds children for every
other visitor. Also chains SqlExpression's Type/TypeMapping into equality on
all three window expression types, and makes the nullability-wrapper unwrap
idempotent (duplicated placeholders no longer leak literal __wf_placeholder_N
into the SQL).

[Copilot]

Pull request overview

This PR fixes a correctness gap in the EF Core relational window-function SQL expression types: WindowFunctionSqlExpression.VisitChildren previously self-rendered in a way that prevented non-SQL-generation visitors (e.g., alias remapping and projection pushdown) from preserving their rewrites. The update keeps self-rendering only for QuerySqlGenerator while ensuring all other visitors rebuild children normally, and it tightens equality/hash semantics to include SqlExpression base state (CLR Type and TypeMapping). It also makes the placeholder unwrap logic idempotent so duplicated placeholders no longer leak into final SQL.

Changes:

• Rework WindowFunctionSqlExpression.VisitChildren to rebuild rewritten children for non-QuerySqlGenerator visitors while retaining self-rendering for SQL generation.
• Include SqlExpression base equality/hash (Type/TypeMapping) in window expression equality across the three window expression types.
• Make window-function placeholder unwrapping idempotent and add targeted tests covering visitor rewrites, equality, and duplicated placeholder restoration.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/ExpressiveSharp.Tests/RelationalExtensions/WindowFunctionSqlExpressionTests.cs	Adds unit tests ensuring visitor rewrites are preserved, equality respects CLR type, and duplicated placeholders unwrap correctly.
src/ExpressiveSharp.EntityFrameworkCore.RelationalExtensions/Infrastructure/Internal/WindowSpecSqlExpression.cs	Extends equality/hash to include SqlExpression base state (Type/TypeMapping).
src/ExpressiveSharp.EntityFrameworkCore.RelationalExtensions/Infrastructure/Internal/WindowFunctionSqlExpressionWrapper.cs	Makes placeholder unwrapping non-destructive so the same placeholder can be restored multiple times.
src/ExpressiveSharp.EntityFrameworkCore.RelationalExtensions/Infrastructure/Internal/WindowFunctionSqlExpression.cs	Updates VisitChildren to preserve rewrite results for non-SQL visitors; extends equality/hash to include base state.
src/ExpressiveSharp.EntityFrameworkCore.RelationalExtensions/Infrastructure/Internal/WindowFrameBoundSqlExpression.cs	Extends equality/hash to include SqlExpression base state (Type/TypeMapping).

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[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...tructure/Internal/WindowFrameBoundSqlExpression.cs	50.00%	0 Missing and 1 partial ⚠️
...astructure/Internal/WindowFunctionSqlExpression.cs	96.00%	0 Missing and 1 partial ⚠️

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