fix(subscriptions): use x-api-key connectionParams format for WebSocket auth

The Unraid graphql-ws server expects the API key directly in connectionParams
as `x-api-key`, not nested under `headers`. The old format caused the server
to fall through to cookie auth and crash on `undefined.csrf_token`.

Fixed in snapshot.py (×2), manager.py, diagnostics.py, and updated the
integration test assertion to match the correct payload shape.

unraid_mcp/subscriptions/diagnostics.py

@@ -130,7 +130,7 @@ def register_diagnostic_tools(mcp: FastMCP) -> None:
                     json.dumps(
                         {
                             "type": "connection_init",
-                            "payload": {"headers": {"X-API-Key": UNRAID_API_KEY}},
+                            "payload": {"x-api-key": UNRAID_API_KEY},
                         }
                     )
                 )