ensure context cleanup runs even if teardown raises

[bysiber]

In AppContext.pop(), if do_teardown_appcontext raises an exception, _cv_app.reset() and the context token cleanup are skipped. This leaves the context variable in a corrupted state, which can cause subsequent operations to see stale context data.

Similarly, if do_teardown_request raises, _request.close() is not called, potentially leaking file handles from the request body.

This adds nested try/finally blocks to ensure:

• _request.close() runs even if do_teardown_request raises
• _cv_app.reset() and token cleanup run even if do_teardown_appcontext raises
• The appcontext_popped signal is always sent

While teardown functions should avoid raising exceptions per the documentation, the framework-level cleanup should still be guaranteed regardless.