Fix IAsyncEnumerable detection in MethodDataSource source generator

[Copilot]

The source generator was not correctly detecting when a method returns async IAsyncEnumerable<T>, causing MethodDataSource attributes with async enumerable data sources to throw InvalidCastException at runtime.

Problem

When using MethodDataSource with an IAsyncEnumerable, TUnit would attempt to cast the compiler-generated async iterator (e.g., <GetAsyncTestData>d__0) directly to the test data type instead of iterating through the async enumerable and yielding individual values.

// This would fail with InvalidCastException
public static async IAsyncEnumerable<(int, string)> GetSimpleAsyncData()
{
    await Task.Delay(1);
    yield return (1, "first");
    yield return (2, "second");
    yield return (3, "third");
}

[Test]
[MethodDataSource(nameof(GetSimpleAsyncData))]
public async Task TestWithAsyncSimpleData(int id, string name) { ... }

Root Cause

The IsAsyncEnumerable() method in TestMetadataGenerator.cs only checked type.AllInterfaces for IAsyncEnumerable<T> implementations, but didn't check if the type itself was an IAsyncEnumerable<T>. For methods returning async IAsyncEnumerable<T>, the return type is directly IAsyncEnumerable<T>, not a type that implements it.

Solution

Updated the detection logic to check both:

1. If the type itself is IAsyncEnumerable<T> (for direct returns)
2. If the type implements IAsyncEnumerable<T> (for inherited implementations)

This ensures the source generator correctly generates the async enumerable iteration code:

await foreach (var item in result)
{
    yield return () => Task.FromResult(DataSourceHelpers.ToObjectArray(item));
}

Instead of incorrectly treating the async iterator as a single value.

Testing

Added comprehensive test cases covering:

• IAsyncEnumerable<Func<CustomType>> (matching documentation examples)
• IAsyncEnumerable<(int, string)> (tuple returns)
• IAsyncEnumerable<object[]> (existing patterns)

All async enumerable tests now pass, and existing functionality remains unaffected.

Fixes #2990.

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