test: cover getErrorText interpolation of i18n_params

Address pr-swarm finding: design (the fixed line itself was untested).

frontend/src/message/index.test.ts

@@ -0,0 +1,60 @@
+import {describe, it, expect} from 'vitest'
+
+import {getErrorText} from './index'
+
+describe('getErrorText', () => {
+	it('interpolates i18n_params into the translated error message', () => {
+		const text = getErrorText({
+			response: {
+				data: {
+					code: 14002,
+					message: 'The permission frobnicate of group tasks is invalid.',
+					i18n_params: {permission: 'frobnicate', group: 'tasks'},
+				},
+			},
+		})
+
+		expect(text).toContain('frobnicate')
+		expect(text).toContain('tasks')
+	})
+
+	it('falls back to empty placeholders when i18n_params is missing, without crashing', () => {
+		const text = getErrorText({
+			response: {
+				data: {
+					code: 14002,
+					message: 'The permission frobnicate of group tasks is invalid.',
+				},
+			},
+		})
+
+		expect(text).not.toContain('{permission}')
+		expect(text).not.toContain('{group}')
+		expect(text).not.toContain('undefined')
+	})
+
+	it('falls back to data.message when there is no error code', () => {
+		const text = getErrorText({
+			response: {
+				data: {
+					message: 'Something went wrong',
+				},
+			},
+		})
+
+		expect(text).toBe('Something went wrong')
+	})
+
+	it('falls back to data.message for an unknown error code', () => {
+		const text = getErrorText({
+			response: {
+				data: {
+					code: 99999,
+					message: 'some backend message',
+				},
+			},
+		})
+
+		expect(text).toBe('some backend message')
+	})
+})