[ResponseOps][Connectors] Fix missing action variables in Slack Web API text message mode (elastic#259499)

Resolves elastic#259496

<img width="1159" height="724" alt="Screenshot 2026-03-25 at 9 23 24 AM"
src="https://github.com/user-attachments/assets/8eefc9ef-8a76-445a-bdec-d556a4ecd7ef"
/>

Author: benakansara

x-pack/platform/plugins/shared/stack_connectors/public/connector_types/slack_api/slack_params.test.tsx

@@ -417,6 +417,33 @@ describe('SlackParamsFields', () => {
       });
     });
 
+    it('renders action variables in Text mode', async () => {
+      const messageVariables = [
+        {
+          name: 'alert.id',
+          description: 'The alert ID',
+        },
+      ];
+
+      appMockRenderer.render(
+        <SlackParamsFields
+          actionParams={{}}
+          editAction={editAction}
+          index={0}
+          errors={{ message: [] }}
+          actionConnector={actionConnector}
+          messageVariables={messageVariables}
+        />
+      );
+
+      const addVariableButton = screen.getByTestId('webApiTextAddVariableButton');
+      expect(addVariableButton).toBeInTheDocument();
+      expect(addVariableButton).not.toBeDisabled();
+
+      await userEvent.click(addVariableButton);
+      expect(await screen.findByTestId('variableMenuButton-alert.id')).toBeInTheDocument();
+    });
+
     it('cannot set channels not in the allow list', async () => {
       appMockRenderer.render(
         <SlackParamsFields

x-pack/platform/plugins/shared/stack_connectors/public/connector_types/slack_api/slack_params.tsx

@@ -240,6 +240,7 @@ const SlackParamsFieldsComponent: React.FunctionComponent<ParamsProps> = ({
         <TextMessage
           index={index}
           onChange={onTextChange}
+          messageVariables={messageVariables}
           value={textValue}
           errors={(errors.text ?? []) as string[]}
         />