Enhance OPC UA testing framework and documentation
- Added a new marker in pyproject.toml for tests using the in-process OPC UA server.
- Updated opc-communication.md to clarify E2E test scenarios involving the real OPC server and mock server.
- Introduced an in-process asyncua OPC UA server fixture in conftest.py for E2E tests.
- Created a new fixture for activities using the real OpcRepository connected to the in-process server.
- Updated scenarios.md to include instructions for running OPC real-server tests.
