Add support for AggregateException in SkippableFact

[Copilot]

This PR adds support for handling AggregateException in Xunit.SkippableFact. When a test throws an AggregateException containing skip exceptions, the test will now be properly skipped instead of failing.

Problem

Previously, tests that threw AggregateException containing skip exceptions would fail instead of being skipped:

[SkippableFact]
public void TestWithAggregateException()
{
    var skipException = new SkipException("Should skip this test");
    var otherException = new InvalidOperationException("Some error");
    var aggregateException = new AggregateException(skipException, otherException);
    throw aggregateException; // Would FAIL instead of SKIP
}

This was particularly problematic when using Task.WhenAll or similar patterns that aggregate multiple exceptions, where one of the exceptions was a skip condition.

Solution

The implementation adds a new case to SkippableTestMessageBus.QueueMessage() that:

1. Detects when the outer exception is System.AggregateException
2. Iterates through the inner exceptions within the aggregate
3. If any inner exception matches a configured skip exception type, skips the test
4. Uses the message from the first matching skip exception as the skip reason

Examples

All of these scenarios now work correctly:

// Single skip exception in aggregate - SKIPPED
[SkippableFact]
public void SingleSkipInAggregate()
{
    var aggregate = new AggregateException(new SkipException("Reason"));
    throw aggregate;
}

// Mixed exceptions with skip - SKIPPED (skip takes precedence)
[SkippableFact] 
public void MixedExceptionsWithSkip()
{
    var skip = new SkipException("Skip reason");
    var error = new InvalidOperationException("Error");
    throw new AggregateException(skip, error);
}

// Custom skip exception types - SKIPPED
[SkippableFact(typeof(NotImplementedException))]
public void CustomSkipType()
{
    var skip = new NotImplementedException("Not implemented");
    var error = new ArgumentException("Invalid arg");
    throw new AggregateException(skip, error);
}

// No skip exceptions - FAILED (as expected)
[SkippableFact]
public void NoSkipExceptions()
{
    var error1 = new InvalidOperationException("Error 1");
    var error2 = new ArgumentException("Error 2");
    throw new AggregateException(error1, error2); // Still fails
}

Implementation Details

• Minimal changes: Only 13 lines added to core implementation
• Consistent pattern: Follows existing ThrowsException handling approach
• Backward compatible: All existing functionality preserved
• Comprehensive tests: Added 7 test cases covering various scenarios including nested aggregates

The implementation follows the requirement to skip if ANY inner exception is a skip exception (not requiring ALL to be skip exceptions), which aligns with typical test failure scenarios.

Fixes #31.

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