fix(playwright): track and drain in-flight rendering detections on teardown

[Yash-Hirgude]

fix(playwright): track and drain in-flight rendering detections on teardown

• Track active detection promises inside activeDetections
• Expose drainRenderingDetections() to await pending detections
• Automatically drain detections during teardown() before disposing predictor
• Add unit tests for detection tracking and draining lifecycle

Closes #4074