Add aria-label to Custom Instructions textarea in edit details flyout (elastic#271073)

Author: Copilot

x-pack/platform/plugins/shared/agent_builder/public/application/components/agents/overview/edit_details_flyout/custom_instructions_section.tsx

@@ -36,6 +36,7 @@ export const CustomInstructionsSection: React.FC = () => {
             inputRef={ref}
             fullWidth
             rows={6}
+            aria-label={flyoutLabels.instructionsTitle}
             placeholder={flyoutLabels.instructionsPlaceholder}
             data-test-subj="editDetailsInstructionsInput"
           />