Add AssertionResult.Failed overload that accepts an Exception

[thomhurst]

Problem Statement

When inline/nested assertions fail (e.g. inside WithInnerExceptions, Count(c => c.IsEqualTo(5)), All().Satisfy()), the failure is caught and converted to an AssertionResult.Failed(string). This loses the original exception's structured information — stack trace, inner exceptions, and any typed data on the AssertionException.

Currently there are 12+ sites in the codebase that use this pattern:

try
{
    await resultingAssertion.AssertAsync();
}
catch
{
    return AssertionResult.Failed("some message");
}

The original exception is discarded, making it harder for users to diagnose why a nested assertion failed.

Proposed Solution

Add an Exception property to AssertionResult and a new Failed overload:

public readonly struct AssertionResult
{
    public bool IsPassed { get; }
    public string Message { get; }
    public Exception? Exception { get; }

    public static AssertionResult Failed(string message, Exception? exception = null)
        => new(false, message, exception);
}

Then update Assertion<T>.ExecuteCoreAsync / CreateException to surface the inner exception when rendering failure output, and update the 12+ inline assertion catch sites to pass through the caught exception.

Benefits

• Users get full error context when nested assertions fail
• Stack traces are preserved for debugging
• Consistent with how .NET surfaces inner failures (e.g. AggregateException.InnerExceptions)

Context

Discovered while implementing #5345 (WithInnerExceptions). The bare catch pattern is consistent with the existing codebase but loses valuable diagnostic information.

Feature Category

Assertions