Fix #ImplicitStrongCapture warning in generateAppleOnDeviceToolResponse

Rune caught this via a Release build (build.sh). The onWillStart
closure used [weak self] while its enclosing Task closure already
captures self strongly (matching generateAIResponseWithTools's
existing convention, which uses no weak self anywhere) — mixing
capture strength between nested closures over the same self is
exactly what Swift 6's new #ImplicitStrongCapture diagnostic flags.
Not a functional bug (self was already kept alive for the Task's
whole duration regardless), just inconsistent with this file's own
pattern. Removed the pointless weak capture.

oAI/ViewModels/ChatViewModel.swift

@@ -2019,8 +2019,13 @@ Don't narrate future actions ("Let me...") - just use the tools.
             // pattern already proven in AppleDynamicToolTests' CallbackRecorder).
             let collector = ToolCallDetailCollector()
 
-            let onWillStart: AppleToolCallWillStart = { [weak self] toolName in
-                self?.currentToolActivity = String(localized: "🔧 Calling: \(toolName)")
+            // No [weak self] here: the enclosing Task already captures self strongly (same
+            // convention generateAIResponseWithTools uses) — a weak capture in this nested closure
+            // wouldn't protect against anything self isn't already kept alive for, and mixing
+            // strengths between nested closures capturing the same self is what
+            // #ImplicitStrongCapture flags.
+            let onWillStart: AppleToolCallWillStart = { toolName in
+                self.currentToolActivity = String(localized: "🔧 Calling: \(toolName)")
             }
             let onDidFinish: AppleToolCallDidFinish = { detail in
                 collector.details.append(detail)