feat: support AssertionResult<T> in GenerateAssertion for terminal assertions

[thomhurst]

Summary

• Introduces AssertionResult<T> readonly struct for assertions that return a value when awaited (terminal assertions)
• Extends [GenerateAssertion] source generator to recognize AssertionResult<T> and Task<AssertionResult<T>> return types
• Generated assertion classes include _result capture field, new GetAwaiter() override, and ExecuteAndReturnResultAsync() to enable string matched = await Assert.That(items).ContainsMatch("foo")

Closes #4829

Usage

[GenerateAssertion(ExpectationMessage = "to contain '{needle}'")]
public static AssertionResult<string> ContainsMatch(this IEnumerable<string> strings, string needle)
{
    var result = strings.FirstOrDefault(x => x.Contains(needle));
    if (result is not null)
        return AssertionResult<string>.Passed(result);
    return AssertionResult.Failed($"{needle} not found");
}

// Terminal assertion — returns the matched item:
string matched = await Assert.That(items).ContainsMatch("foo");

Test plan

• Source generator snapshot tests for sync AssertionResult<T> return type
• Source generator snapshot tests for async Task<AssertionResult<T>> return type
• Verified generated code includes _result field, new GetAwaiter<T>(), and unwrapping logic
• Existing MethodAssertionGeneratorTests pass without regression
• TUnit.Assertions builds cleanly across all target frameworks (net8.0, net9.0, net10.0, netstandard2.0)

🤖 Generated with Claude Code

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

This PR introduces a well-designed feature for terminal assertions that return typed values. The implementation:

• Properly handles both sync (AssertionResult<T>) and async (Task<AssertionResult<T>>) patterns
• Uses readonly struct for zero-allocation value semantics
• Correctly committed snapshot test files (only .verified.txt)
• Follows existing TUnit patterns consistently
• Maintains type safety with appropriate nullable handling