Remove SqlExpressionVisitor

[roji]

Cleanup/refactor. We currently have SqlExpressionVisitor, whose purpose was to be a visitor base type that forces every kind of SQL expression to be explicitly handled by a visitor. However, this type is only used by QuerySqlGenerator, and there are simpler ways to make sure all expression types are handled (e.g. simply throw for unhandled expressions in VisitExtension). Since this base type adds no real value, this removes it.

Also cleans up some minor things in QuerySqlGenerator. Technically contains provider-facing breaking changes, but these are extremely unlikely to affect anyone in practice.

[Copilot]

Pull request overview

This PR removes the intermediate SqlExpressionVisitor base class and refactors QuerySqlGenerator to inherit directly from ExpressionVisitor. The purpose is to simplify the codebase since SqlExpressionVisitor was only used by QuerySqlGenerator and didn't provide significant value.

Changes:

• Deleted SqlExpressionVisitor.cs entirely (315 lines)
• Refactored QuerySqlGenerator to inherit from ExpressionVisitor instead of SqlExpressionVisitor, moved the switch-based VisitExtension logic directly into the class, and converted to primary constructor syntax
• Changed all Visit method signatures from protected override to protected virtual and made VisitLike, VisitExists, and VisitIn private since they no longer need to be overridable

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/EFCore.Relational/Query/SqlExpressionVisitor.cs	Complete removal of the intermediate visitor base class
src/EFCore.Relational/Query/QuerySqlGenerator.cs	Refactored to inherit directly from ExpressionVisitor, moved VisitExtension logic inline, converted to primary constructor, changed method signatures appropriately, and removed obsolete method