test(contract): tidy the purge provider-state no-op stub

Round-2 style note: replace `return None` with a comment-only intentionally
empty body for the _state_admin_can_purge stub.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/contract/test_mcp_provider_verification.py

@@ -81,7 +81,7 @@ def _state_admin_can_purge() -> None:
     is registered so the dispatcher recognises the state by name rather than
     logging an "unhandled state" warning.
     """
-    return None
+    # Intentionally empty: no live-stack state to set up yet (phase 4).
 
 
 _PROVIDER_STATES: dict[str, Callable[[], None]] = {