Add a new session-scoped placeholder fixture (nmstate_dependent_placeholder) that serves as a dependency marker for fixtures interacting with NMState Custom Resources (NNCP, NNCE, NNS).

Mark all identified NMState-dependent fixtures with @pytest.mark.usefixtures("nmstate_dependent_placeholder") to establish the dependency graph.

This change establishes the foundation for the new collection logic that will be implemented in pytest_collection_modifyitems. The collection logic will traverse the fixture dependency raph to automatically detect and mark tests that require NMState functionality, enabling conditional NMState verification in network_sanity() and cloud-compatible test execution.