feat(companion): expose heartbeat loop failure observability

src/companion/heartbeatLoop.test.ts

@@ -109,9 +109,13 @@ describe('CompanionHeartbeatLoop', () => {
     loop.start();
     await Promise.resolve();
     expect(onError).toHaveBeenCalledTimes(1);
+    expect(loop.failureCount).toBe(1);
+    expect(loop.lastFailure?.message).toBe('boom');
 
     await vi.advanceTimersByTimeAsync(400);
     expect(publishHeartbeat).toHaveBeenCalledTimes(2);
+    expect(loop.failureCount).toBe(0);
+    expect(loop.lastFailure).toBeNull();
 
     loop.stop();
   });
@@ -154,6 +158,8 @@ describe('CompanionHeartbeatLoop', () => {
 
     await vi.advanceTimersByTimeAsync(300);
     expect(loop.running).toBe(false);
+    expect(loop.failureCount).toBe(2);
+    expect(loop.lastFailure?.message).toBe('persistent-failure');
     expect(onError).toHaveBeenCalledTimes(2);
     expect(onFailureLimitReached).toHaveBeenCalledTimes(1);
     expect(onFailureLimitReached).toHaveBeenCalledWith(expect.any(Error), 2);