Add Expression<Func<TSource, TTarget>> mapping support

Description

Adds support for generating Expression<Func<TSource, TTarget>> mappings directly:

[Mapper]
public static partial class CarMapper
{
    public static partial Expression<Func<Car, CarDto>> ProjectToDto();
}

Implementation

• UserDefinedExpressionMethodMapping - Handles parameterless methods returning Expression, extracting source/target from type arguments
• ExpressionMapping - Generates lambda with nullable pragmas (same pattern as IQueryable)
• Reuses InlineExpressionMappingBuilderContext for expression tree building
• No changes to IQueryable behavior

vs #951

PR #951 modified IQueryable to call a generated Expression method, requiring a "void source hack" throughout core classes.
This PR keeps Expression mappings separate from IQuaryable. They share implementation details, but still separate branches.
"void source hack" is still present here, but I have tried to keep it's impact limited to UserDefinedExpressionMethodMapping type only.

Fixes #733

Checklist

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• The documentation is updated (as applicable)
• Unit tests are added/updated
• Integration tests are added/updated (as applicable, especially if feature/bug depends on roslyn or framework version in use)