fix: update BackgroundTasksIdleTests for refactored handler pattern

PR #526 extracted SubagentDeferStartedAtTicks stamping into
RefreshDeferredBackgroundTaskTracking() and changed the case pattern
to use a named variable. Update the source-scanning tests:
- Remove trailing colon from case pattern search (handler uses
  pattern variable now)
- Accept RefreshDeferredBackgroundTaskTracking call as valid
  stamping mechanism (uses Interlocked.Exchange internally)

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

Author: PureWeen

PolyPilot.Tests/BackgroundTasksIdleTests.cs

@@ -228,17 +228,20 @@ public void ProactiveIdleDefer_SubagentDeferStartedAtTicks_StampedOnBackgroundTa
         var source = File.ReadAllText(Path.Combine(GetRepoRoot(),
             "PolyPilot", "Services", "CopilotService.Events.cs"));
 
-        var handlerStart = source.IndexOf("case SessionBackgroundTasksChangedEvent:");
+        var handlerStart = source.IndexOf("case SessionBackgroundTasksChangedEvent");
         Assert.True(handlerStart >= 0, "SessionBackgroundTasksChangedEvent handler not found");
 
         // Find the next case or closing brace to bound the handler
         var handlerEnd = source.IndexOf("case System", handlerStart + 1);
         if (handlerEnd < 0) handlerEnd = source.Length;
         var handler = source.Substring(handlerStart, handlerEnd - handlerStart);
 
-        // Must stamp SubagentDeferStartedAtTicks via CompareExchange
-        Assert.Contains("SubagentDeferStartedAtTicks", handler);
-        Assert.Contains("CompareExchange", handler);
+        // Must stamp SubagentDeferStartedAtTicks — either directly via atomic ops
+        // or indirectly via RefreshDeferredBackgroundTaskTracking (which uses Interlocked.Exchange)
+        Assert.True(
+            handler.Contains("SubagentDeferStartedAtTicks") ||
+            handler.Contains("RefreshDeferredBackgroundTaskTracking"),
+            "Handler must stamp SubagentDeferStartedAtTicks directly or via RefreshDeferredBackgroundTaskTracking");
     }
 
     [Fact]
@@ -249,7 +252,7 @@ public void ProactiveIdleDefer_Handler_DoesNotSetIsProcessingFalse()
         var source = File.ReadAllText(Path.Combine(GetRepoRoot(),
             "PolyPilot", "Services", "CopilotService.Events.cs"));
 
-        var handlerStart = source.IndexOf("case SessionBackgroundTasksChangedEvent:");
+        var handlerStart = source.IndexOf("case SessionBackgroundTasksChangedEvent");
         Assert.True(handlerStart >= 0, "SessionBackgroundTasksChangedEvent handler not found");
 
         var handlerEnd = source.IndexOf("case System", handlerStart + 1);