fix(macos): remove self-delegate on cost usage submenu to prevent recursive dropdown

The cost usage submenu set `menu.delegate = self` (the MenuSessionsInjector),
which caused `menuWillOpen(_:)` to call `inject(into:)` on the submenu when
it opened. This re-inserted the "Usage cost (30 days)" item into the submenu,
creating an infinite recursive dropdown.

Fix: remove the delegate assignment from the submenu — it does not need
the injector's delegate behavior since it only contains a static chart view.

Closes #25167

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/macos/Sources/OpenClaw/MenuSessionsInjector.swift

@@ -493,7 +493,6 @@ extension MenuSessionsInjector {
         guard !summary.daily.isEmpty else { return nil }
 
         let menu = NSMenu()
-        menu.delegate = self
 
         let chartView = CostUsageHistoryMenuView(summary: summary, width: width)
         let hosting = NSHostingView(rootView: AnyView(chartView))