fix: gate scrollbar resize guard on size, preserve parse causes

Address CodeRabbit review on #6510:
- gridstack: only suppress recalculation when the width delta is
  insignificant AND (non-dynamic OR height unchanged), so dynamic
  sections still rebuild on height changes
- beszel: propagate the response parse failure via { cause } in both
  error wrappers
- auth tests: cover invalid (non-string) OIDC groups claims

packages/integrations/src/beszel/beszel-integration.ts

@@ -171,9 +171,11 @@ export class BeszelIntegration extends Integration {
         integrationId: this.integration.id,
         error: error instanceof Error ? error.message : String(error),
       });
-      throw new ErrorWithMetadata("Beszel authentication response was not valid JSON", {
-        integrationId: this.integration.id,
-      });
+      throw new ErrorWithMetadata(
+        "Beszel authentication response was not valid JSON",
+        { integrationId: this.integration.id },
+        { cause: error instanceof Error ? error : new Error(String(error)) },
+      );
     }
     const expiresAt = parseTokenExpiration(data.token) ?? undefined;
 
@@ -626,7 +628,11 @@ export class BeszelIntegration extends Integration {
       logger.warn("Failed to parse Beszel testing response JSON", {
         error: error instanceof Error ? error.message : String(error),
       });
-      return TestConnectionError.ParseResult(new ParseError("Beszel testing response was not valid JSON"));
+      return TestConnectionError.ParseResult(
+        new ParseError("Beszel testing response was not valid JSON", {
+          cause: error instanceof Error ? error : new Error(String(error)),
+        }),
+      );
     }
     if (!data.token) {
       return TestConnectionError.UnauthorizedResult(401);