Fix: Prevent changing void to Task for event handler methods in await completion

Issue Summary

Regression where typing await in an event handler method changed the return type from void to Task, making the code uncompilable. Event handlers in C# must have void return type.

Before (BROKEN):

private void OnMyEvent(object sender, EventArgs e) { await ... }
// Became: private async Task OnMyEvent(...) ❌ Compile error!

After (FIXED):

private void OnMyEvent(object sender, EventArgs e) { await ... }
// Becomes: private async void OnMyEvent(...) ✅ Correct!

Solution Implemented

• Added detection logic to identify when a method is used as an event handler
• Modified return type change logic to preserve void for event handlers
• Added comprehensive integration tests (8 test cases in CSharpCompletionCommandHandlerTests_Await.vb)
• Added WorkItem attribute to all tests for issue tracking
• Built and verified all affected projects
• Ran code formatting (no changes needed)
• Ran CodeQL security check (passed)

Detection Logic

The fix searches the containing type for patterns like:

• MyEvent += OnMyEvent (event registration)
• MyEvent -= OnMyEvent (event unregistration)

When detected, it:

1. Adds the async modifier (required for await)
2. Preserves the void return type (required for event handlers)
3. Still changes void → Task for non-event-handler methods

Test Coverage

All tests are integration tests in CSharpCompletionCommandHandlerTests_Await.vb:

Event Handler Cases (void preserved):

• Standard EventHandler delegate
• Custom delegate types
• Registration in constructor or other methods
• Both += and -= operators

Non-Event Handler Cases (void → Task):

• Regular void methods without event registration

Build Results

✅ All projects build successfully with 0 warnings
✅ No security vulnerabilities detected

Files Changed

• AwaitCompletionProvider.cs (+55 lines)
• CSharpCompletionCommandHandlerTests_Await.vb (+257 lines) - all integration tests with WorkItem attributes

Total: 312 lines added, 2 files modified

Fixes #77848

• Fixes C# editor breaking change in behavior when writing await keyword inside event handler #77848

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