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Python: add context manager compatibility for VideoCapture and VideoWriter #28967 ### Description This PR implements the context manager protocol (`with` statement support) for `cv2.VideoCapture` and `cv2.VideoWriter` objects in Python. ### Problem Currently, these classes do not support the context manager protocol, leading to a `TypeError: ... object does not support the context manager protocol` when used with a `with` block. As reported in issue #28956, if an exception occurs before an explicit `.release()` call, the video resource or file remains locked in the system[cite: 298, 309]. ### Identification & Motivation By analyzing the issue and the Python binding structure of OpenCV, I identified that while the core logic resides in C++, the Pythonic behavior can be enhanced by injecting `__enter__` and `__exit__` methods during the module initialization. This ensures that resources are automatically and safely released even if the program execution is interrupted by an error[cite: 298, 325]. ### Solution I have modified `modules/python/package/cv2/__init__.py` to dynamically inject the necessary methods: 1. Defined `_video_enter` to return the object itself (standard context manager behavior). 2. Defined `_video_exit` to trigger the `.release()` method automatically. 3. Injected these methods into `VideoCapture` and `VideoWriter` classes immediately after the `bootstrap()` function to ensure the native C++ classes are correctly loaded into the global namespace. Fixes #28956
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