CMTK0003 misses async assertions inside rethrowing catches #166

Problem

CMTK0003 treats every catch surrounding an awaited assertion in an async lambda as if it observed the failure. A catch that unconditionally rethrows, cannot catch the assertion exception, or has an always-false filter does not observe it; the outer dropped task remains the only place that failure is visible.

Reproduced at release tip 9878a6cb7500dbf7d3e1a199c3b9446d7e5cc278 with the real analyzer and Testing assemblies through Roslyn Compilation.WithAnalyzers, without compiler errors:

public static void Check(Task<Option<int>> pending)
{
    Task.WhenAll(new[] { pending }.Select(async item =>
    {
        try { await item.ShouldBeNone(); }
        catch { throw; }
    }));
}

No CMTK0003 is produced. Replacing the catch with catch (OperationCanceledException) { } or catch (Exception) when (false) { } also gives no diagnostic. All three leave a TypeKitAssertionException unobserved when the pending value is Some. A runtime check confirmed the rethrowing lambda faults its outer WhenAll task with TypeKitAssertionException; dropping that task lets the test pass.

Controls: a directly awaited assertion in a lambda without a catch (including a try/finally) is diagnosed when the outer WhenAll task is dropped. A catch that actually swallows the failure is correctly silent.

Cause

IgnoredOutcomeAnalyzer.EnclosingAsyncFunction immediately returns null for any ITryOperation with a nonempty Catches list, without considering whether those catches handle the failure or rethrow it. HandsOn therefore loses the assertion's failure path before it reaches the async lambda and task combinator.

Expected

Do not treat a rethrow-only catch or a catch that cannot handle the assertion's failure as observing it. Extend the async-lambda axis of DroppedTaskShapes with no catch, try/finally, swallowing catch, rethrow/log-and-rethrow, unrelated exception type and filtered catch controls. Keep awaited/held/returned/explicit-discard uses silent where appropriate.

Found in the second pre-release review, in the async-lambda reach added with #157/#164. Decide and fix before the tag under decision 119.