Add tests for using the mock as a decorator - #3450

MockVWS is a ContextDecorator, but every existing test used it only as a context manager. This adds a TestDecorator class to tests/mock_vws/test_requests_mock_usage.py covering decorator usage: requests and httpx requests are mocked inside the decorated function and unmocked after it returns, arguments and return values pass through, the function's name and docstring are preserved, databases and options (such as processing_time_seconds) given to the mock instance apply inside, targets persist across repeated calls of the same decorated function, a full add-then-query flow works, and methods can be decorated.

The tests live in an existing test module rather than a new file because CI shards tests by an explicit ci_pattern matrix whose entry count matches the number of per-job credential files in secrets.tar.gpg.

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