M8-R20: enforce warning-free frontend tests

frontend/src/test-warning-guard.test.ts

@@ -0,0 +1,40 @@
+import { describe, expect, it, vi } from 'vitest'
+import {
+  assertNoReactTestWarnings,
+  createReactWarningGuard,
+  formatReactTestWarnings,
+  isReactTestWarning,
+} from './test-warning-guard'
+
+describe('React test warning guard', () => {
+  it('recognizes React act and duplicate-key warnings only', () => {
+    expect(isReactTestWarning(['Warning: An update to Example inside a test was not wrapped in act(...).'])).toBe(true)
+    expect(isReactTestWarning(['Warning: Each child in a list should have a unique "key" prop.'])).toBe(true)
+    expect(isReactTestWarning(['Warning: Encountered two children with the same key, `duplicate`.'])).toBe(true)
+    expect(isReactTestWarning(['network request failed'])).toBe(false)
+  })
+
+  it('collects target warnings for one compact diagnostic and forwards unrelated errors', () => {
+    const originalConsoleError = vi.fn()
+    const warnings: unknown[][] = []
+    const guardedConsoleError = createReactWarningGuard(originalConsoleError, warnings)
+
+    guardedConsoleError('Warning: An update to Example inside a test was not wrapped in act(...)')
+    guardedConsoleError('Warning: An update to Other inside a test was not wrapped in act(...)')
+    guardedConsoleError('Warning: Encountered two children with the same key, `duplicate`.')
+    guardedConsoleError('network request failed', { status: 500 })
+
+    expect(originalConsoleError).toHaveBeenCalledTimes(1)
+    expect(originalConsoleError).toHaveBeenCalledWith('network request failed', { status: 500 })
+    expect(formatReactTestWarnings(warnings)).toBe(
+      'React act warning: 2, React duplicate-key warning: 1 (Example, Other)',
+    )
+    expect(() => assertNoReactTestWarnings(warnings)).toThrow(
+      'React test warning guard: React act warning: 2, React duplicate-key warning: 1 (Example, Other).',
+    )
+  })
+
+  it('passes only when no target warnings were captured', () => {
+    expect(() => assertNoReactTestWarnings([])).not.toThrow()
+  })
+})