Document CmdPal lifecycle contracts

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 7a42451c-0ec4-4778-86fd-c77d7ac9d122

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Services/ExtensionTaskCoordinator.cs

@@ -51,6 +51,9 @@ internal static class ExtensionTaskCoordinator
         });
 
         var results = await Task.WhenAll(tasks).ConfigureAwait(false);
+
+        // Cancellation is not partial success. The current reload may stop here, and the next
+        // reload stops every service before rediscovering all providers.
         cancellationToken.ThrowIfCancellationRequested();
         return results.OfType<TResult>().ToArray();
     }

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Services/JsonRpcExtensionService.cs

@@ -82,6 +82,9 @@ public sealed partial class JsonRpcExtensionService : IExtensionService, IDispos
     private static readonly string ExtensionsPath = GetDefaultExtensionsPath();
 
     private readonly TaskScheduler _taskScheduler;
+
+    // This gate lives for the service lifetime. Disposing it while a start owns a permit
+    // could make its matching Release throw.
     private readonly SemaphoreSlim _extensionStartupGate = new(MaxConcurrentExtensionStarts, MaxConcurrentExtensionStarts);
     private readonly Lock _extensionsLock = new();
     private readonly List<JSExtensionWrapper> _extensions = [];