Add Join/LeftJoin/RightJoin tuple overloads to LINQ APIs

[Copilot]

Implements tuple-returning overloads for Join, LeftJoin, and RightJoin that eliminate the need for a resultSelector lambda when you just want the joined elements as a tuple.

Changes

• System.Linq.Enumerable: Added Join<TOuter, TInner, TKey>, LeftJoin<TOuter, TInner, TKey>, RightJoin<TOuter, TInner, TKey> returning (TOuter Outer, TInner Inner) tuples (with nullable element for outer joins)
• System.Linq.Queryable: Added corresponding overloads with Expression<Func<>> key selectors
• System.Linq.AsyncEnumerable: Added overloads for both sync and async key selector variants
• All methods use a single overload with an optional IEqualityComparer<TKey>? comparer = null parameter

Example

Before:

foreach (var (s, pair) in keys.Join(dict, k => k, p => p.Value, (outer, inner) => (outer, inner)))
    Console.WriteLine(s + " : " + pair.Key);

After:

foreach (var (s, pair) in keys.Join(dict, k => k, p => p.Value))
    Console.WriteLine(s + " : " + pair.Key);

Fixes #108799

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[API Proposal]: Linq Join return tuple similar to Zip</issue_title>
<issue_description>### Background and motivation

For simplicity of Join it should just return (TOuter,TInner) instead of the need for resultSelector

API Proposal

namespace System.Linq;

public static class Enumerable
{
    public static IEnumerable<(TOuter Outer, TInner Inner)> Join<TOuter, TInner, TKey>(
        this IEnumerable<TOuter> outer,
        IEnumerable<TInner> inner,
        Func<TOuter, TKey> outerKeySelector,
        Func<TInner, TKey> innerKeySelector,
        IEqualityComparer<TKey>? comparer = null);

    public static IEnumerable<(TOuter Outer, TInner? Inner)> LeftJoin<TOuter, TInner, TKey>(
        this IEnumerable<TOuter> outer,
        IEnumerable<TInner> inner,
        Func<TOuter, TKey> outerKeySelector,
        Func<TInner, TKey> innerKeySelector,
        IEqualityComparer<TKey>? comparer = null);

    public static IEnumerable<(TOuter? Outer, TInner Inner)> RightJoin<TOuter, TInner, TKey>(
        this IEnumerable<TOuter> outer,
        IEnumerable<TInner> inner,
        Func<TOuter, TKey> outerKeySelector,
        Func<TInner, TKey> innerKeySelector,
        IEqualityComparer<TKey>? comparer = null);
}

public static class Queryable
{
    public static IQueryable<(TOuter Outer, TInner Inner)> Join<TOuter, TInner, TKey>(
        this IQueryable<TOuter> outer,
        IEnumerable<TInner> inner,
        Expression<Func<TOuter, TKey>> outerKeySelector,
        Expression<Func<TInner, TKey>> innerKeySelector,
        IEqualityComparer<TKey>? comparer = null);

    public static IQueryable<(TOuter Outer, TInner? Inner)> LeftJoin<TOuter, TInner, TKey>(
        this IQueryable<TOuter> outer,
        IEnumerable<TInner> inner,
        Expression<Func<TOuter, TKey>> outerKeySelector,
        Expression<Func<TInner, TKey>> innerKeySelector,
        IEqualityComparer<TKey>? comparer = null);

    public static IQueryable<(TOuter? Outer, TInner Inner)> RightJoin<TOuter, TInner, TKey>(
        this IQueryable<TOuter> outer,
        IEnumerable<TInner> inner,
        Expression<Func<TOuter, TKey>> outerKeySelector,
        Expression<Func<TInner, TKey>> innerKeySelector,
        IEqualityComparer<TKey>? comparer = null);
}

public static class AsyncEnumerable
{
    public static IAsyncEnumerable<(TOuter Outer, TInner Inner)> Join<TOuter, TInner, TKey>(
        this IAsyncEnumerable<TOuter> outer, 
        IAsyncEnumerable<TInner> inner,
        Func<TOuter, CancellationToken, ValueTask<TKey>> outerKeySelector,
        Func<TInner, CancellationToken, ValueTask<TKey>> innerKeySelector,
        IEqualityComparer<TKey>? comparer = null);

    public static IAsyncEnumerable<(TOuter Outer, TInner Inner)> Join<TOuter, TInner, TKey>(
        this IAsyncEnumerable<TOuter> outer,
        IAsyncEnumerable<TInner> inner,
        Func<TOuter, TKey> outerKeySelector,
        Func<TInner, TKey> innerKeySelector,
        IEqualityComparer<TKey>? comparer = null);

    public static IAsyncEnumerable<(TOuter Outer, TInner? Inner)> LeftJoin<TOuter, TInner, TKey>(
        this IAsyncEnumerable<TOuter> outer, 
        IAsyncEnumerable<TInner> inner,
        Func<TOuter, CancellationToken, ValueTask<TKey>> outerKeySelector,
        Func<TInner, CancellationToken, ValueTask<TKey>> innerKeySelector,
        IEqualityComparer<TKey>? comparer = null);

    public static IAsyncEnumerable<(TOuter Outer, TInner? Inner)> LeftJoin<TOuter, TInner, TKey>(
        this IAsyncEnumerable<TOuter> outer,
        IAsyncEnumerable<TInner> inner,
        Func<TOuter, TKey> outerKeySelector,
        Func<TInner, TKey> innerKeySelector,
        IEqualityComparer<TKey>? comparer = null);

    public static IAsyncEnumerable<(TOuter? Outer, TInner Inner)> RightJoin<TOuter, TInner, TKey>(
        this IAsyncEnumerable<TOuter> outer, 
        IAsyncEnumerable<TInner> inner,
        Func<TOuter, CancellationToken, ValueTask<TKey>> outerKeySelector,
        Func<TInner, CancellationToken, ValueTask<TKey>> innerKeySelector,
        IEqualityComparer<TKey>? comparer = null);

    public static IAsyncEnumerable<(TOuter? Outer, TInner Inner)> RightJoin<TOuter, TInner, TKey>(
        this IAsyncEnumerable<TOuter> outer,
        IAsyncEnumerable<TInner> inner,
        Func<TOuter, TKey> outerKeySelector,
        Func<TInner, TKey> innerKeySelector,
        IEqualityComparer<TKey>? comparer = null);
}

API Usage

var keys = new[] { "x" ,"y" ,"z" };
var dict = new Dictionary<int,string>();

foreach (var (s,pair) in keys.Join(dict,(key) => key,(pair) => pair.Value))
    Console.WriteLine(s + " : " pair.Key);

Alternative Designs

Without this it need to make another lambda just for return tuple

foreach (var (s,pair) in keys.Join(dict,(key) => key,(pair) => pair.Value,(outer,inner) => (outer,inner)))
    Console.WriteLine(s + " : " pair.Key);
```...

</details>

- Fixes dotnet/runtime#120596

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[eiriktsarpalis]

@copilot Please remove this overload, replacing it with a single overload that has an optional IEqualitComparer parameter.

[eiriktsarpalis]

@copilot check that this is the case with all overloads across projects as well.

[Copilot]

Done in 7f53b70. Consolidated the two overloads into a single overload with IEqualityComparer<TKey>? comparer = null for all Join, LeftJoin, and RightJoin tuple-returning methods in Enumerable and Queryable.

[Copilot]

Verified - all tuple-returning overloads across all three projects use the single overload with optional IEqualityComparer<TKey>? comparer = null pattern:

• System.Linq: Join, LeftJoin, RightJoin (1 overload each)
• System.Linq.Queryable: Join, LeftJoin, RightJoin (1 overload each)
• System.Linq.AsyncEnumerable: Join, LeftJoin, RightJoin (2 overloads each - one for sync key selectors, one for async key selectors, both with comparer = null)