feat(companion): expose runtime event subscription count

src/companion/runtimeClient.test.ts

@@ -110,7 +110,9 @@ describe('CompanionRuntimeClient', () => {
       url: 'ws://127.0.0.1:1',
     });
     const handler = vi.fn();
+    expect(client.eventSubscriptionCount).toBe(0);
     const unsubscribe = client.subscribeEvents(handler);
+    expect(client.eventSubscriptionCount).toBe(1);
 
     (client as unknown as { handleMessage: (raw: string) => void }).handleMessage(
       JSON.stringify({
@@ -133,6 +135,7 @@ describe('CompanionRuntimeClient', () => {
     );
 
     expect(handler).toHaveBeenCalledTimes(1);
+    expect(client.eventSubscriptionCount).toBe(0);
   });
 
   it('isolates subscriber callback failures', () => {