feat(backend): support federation-based remote node diagnosis

Route diagnosis for shared remote nodes through federation runtime APIs so tunnel and forward diagnostics work across panels, and add contract coverage for single-panel and dual-panel scenarios.

go-backend/internal/http/middleware/auth.go

@@ -91,6 +91,8 @@ func shouldSkip(path string) bool {
 		return true
 	case path == "/api/v1/federation/runtime/release-role":
 		return true
+	case path == "/api/v1/federation/runtime/diagnose":
+		return true
 	default:
 		return false
 	}