Fix nested SelectExpr inconsistency between playground and source generator

[Copilot]

The playground was generating interceptors for nested SelectExpr calls when only the outermost should generate one, and incorrectly typed nested collection properties as ItemDto instead of IEnumerable<ItemDto>.

Changes

Core Infrastructure

• Added IsNestedInsideAnotherSelectExpr() to SelectExprHelper - walks syntax tree to detect nesting

Playground

• Updated CodeGenerationService.IsSelectExprInvocation() to skip nested calls using shared helper

Source Generator

• Refactored SelectExprGenerator.IsSelectExprInvocation() to use shared helper, removing duplicate logic

Tests

• Added NestedSelectExprTypeTest - verifies correct IEnumerable<TDto> type generation
• Added Issue_NestedSelectExprConsistencyTest - validates single interceptor and correct conversion

Example

query.SelectExpr<Entity, EntityDto>(x => new
{
    x.Id,
    ItemTitles = x.Items.SelectExpr<Item, ItemDto>(i => new { i.Title }),
});

Before: Generated two interceptors; ItemTitles typed as ItemDto
After: Generates one interceptor; inner SelectExpr converts to .Select(); ItemTitles typed as IEnumerable<ItemDto>

Original prompt

---

This section details on the original issue you should resolve

<issue_title>bug: When there is a nested SelectExpr, the output in Playground does not match the actual generated result</issue_title>
<issue_description>## Repro

using System;
using System.Linq;
using System.Collections.Generic;
namespace MinRepro;

public class Program
{
    public void Execute()
    {
        var data = new List<Entity>();
        var query = data.AsQueryable();
        // Reproduce your issue here
        var result = query.SelectExpr<Entity, EntityDto>(x => new
        {
            x.Id,
            x.Name,
            ChildDescription = x.Child?.Description,
            ItemTitles = x.Items.SelectExpr<Item, ItemDto>(i => new {i.Title}),
        });
    }
}

// here are some sample classes to work with 
public class Entity
{
    public int Id { get; set; }
    public string Name { get; set; } = "";
    public Child? Child { get; set; }
    public List<Item> Items { get; set; } = [];
}

public class Child
{
    public string Description { get; set; } = "";
}

public class Item
{
    public string Title { get; set; } = "";
}

Actual

in real

// <auto-generated>
// This file is auto-generated by Linqraft.
//   Linqraft Version  : 0.7.0-alpha.8+19d8834e72
//   Linqraft Build at : 2025-12-06 21:57:55
//   Code Generated at : 2025-12-06 22:47:27
// </auto-generated>
#nullable enable
#pragma warning disable IDE0060
#pragma warning disable CS8601
#pragma warning disable CS8602
#pragma warning disable CS8603
#pragma warning disable CS8604
#pragma warning disable CS8618
using System;
using System.Linq;
using System.Collections.Generic;
using System.Collections.Immutable;
namespace Linqraft
{
    file static partial class GeneratedExpression
    {
        /// <summary>
        /// generated select expression method EntityDto (explicit) <br/>
        /// at Program.cs(14,28)
        /// </summary>
        [global::System.Runtime.CompilerServices.InterceptsLocationAttribute(1, "vFxqDeERynAiImVfoKmsVB8BAABQcm9ncmFtLmNz")]
        public static IQueryable<TResult> SelectExpr_FF795AB1_553BA9DB<TIn, TResult>(
            this IQueryable<TIn> query, Func<TIn, object> selector)
        {
            var matchedQuery = query as object as IQueryable<global::MinRepro.Entity>;
            var converted = matchedQuery.Select(x => new global::MinRepro.EntityDto
            {
                Id = x.Id,
                Name = x.Name,
                ChildDescription = x.Child != null ? (string?)x.Child.Description : null,
                ItemTitles = x.Items
                    .Select(i => new global::MinRepro.ItemDto
                    {
                        Title = i.Title
                    })
            });
            return converted as object as IQueryable<TResult>;
        }

    }
}
namespace MinRepro
{
    /// <remarks>
    /// From: <c>Item</c>
    /// </remarks>
    public partial class ItemDto
    {
        /// <remarks>
        /// From: <c>Item.Title</c>
        /// </remarks>
        public required string Title { get; set; }
    }

    /// <summary>
    /// here are some sample classes to work with
    /// </summary>
    /// <remarks>
    /// From: <c>Entity</c>
    /// </remarks>
    public partial class EntityDto
    {
        /// <remarks>
        /// From: <c>Entity.Id</c>
        /// </remarks>
        public required int Id { get; set; }
        /// <remarks>
        /// From: <c>Entity.Name</c>
        /// </remarks>
        public required string Name { get; set; }
        /// <remarks>
        /// From: <c>Entity.Child?.Description</c>
        /// </remarks>
        public required string? ChildDescription { get; set; }
        /// <remarks>
        /// From: <c>Entity.Items.SelectExpr&lt;Item,ItemDto&gt;(i=&gt;new{i.Title})</c>
        /// </remarks>
        public required global::System.Collections.Generic.IEnumerable<MinRepro.ItemDto> ItemTitles { get; set; }
    }
}

in playground

// expression
using System;
using System.Linq;
using System.Collections.Generic;
namespace Linqraft
{
    file static partial class GeneratedExpression
    {
        /// <summary>
        /// generated select expression method EntityDto (explicit) <br/>
        /// at Program.cs(13,28)
        /// </summary>
        [global::System.Runtime.CompilerServices.InterceptsLocationAttribute(1, "uBbZSspQHldj+5NRaGts/B4BAABQcm9ncmFtLmNz")]
        public static IQueryable<TResult> SelectExpr_620CA1C9_0B30C4CE<TIn, TResult>(
            this IQueryable<TIn> query, Func<TIn, object> selector)
        {
            var matchedQuery = query as object as IQueryable<global::MinRepro.Entity>;
            var converted = matchedQuery.Select(x => new global::MinRepro.EntityDto
            {
                Id = x.Id,
                Name = x.Name,
                ChildDescription = x.Child != null ? (string)x.Child.Description : null,
                ItemTitles = x.Items
                   ...

</details>



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- Fixes arika0093/Linqraft#220

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