Fix one-off error in secret variable length check (#32386)

For #32381.

frontend/pages/ManageControlsPage/Secrets/Secrets.tests.tsx

@@ -305,12 +305,12 @@ describe("Custom variables", () => {
       });
       it("does not allow saving very long name", async () => {
         const { nameInput, valueInput, saveButton } = await getAddSecretUI();
-        await user.type(nameInput, new Array(300).fill("A").join("")); // Invalid name
+        await user.type(nameInput, new Array(256).fill("A").join("")); // Invalid name
         await user.type(valueInput, "a value");
         await user.click(saveButton);
         await waitFor(() => {
           expect(
-            screen.getByText("Name may not exceed 256 characters")
+            screen.getByText("Name may not exceed 255 characters")
           ).toBeInTheDocument();
           expect(saveButton).toBeDisabled();
         });

frontend/pages/ManageControlsPage/Secrets/components/AddSecretModal/helpers.ts

@@ -53,9 +53,9 @@ const FORM_VALIDATIONS: IFormValidations = {
       {
         name: "notTooLong",
         isValid: (formData: IAddSecretModalScheduleFormData) => {
-          return formData.name.length <= 256;
+          return formData.name.length <= 255;
         },
-        message: "Name may not exceed 256 characters",
+        message: "Name may not exceed 255 characters",
       },
       {
         name: "doesNotIncludePrefix",