feat(modelAvailabilityService): integrate model availability service into backend logic (#14470)

packages/core/src/availability/testUtils.ts

@@ -0,0 +1,30 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { vi } from 'vitest';
+import type {
+  ModelAvailabilityService,
+  ModelSelectionResult,
+} from './modelAvailabilityService.js';
+
+/**
+ * Test helper to create a fully mocked ModelAvailabilityService.
+ */
+export function createAvailabilityServiceMock(
+  selection: ModelSelectionResult = { selectedModel: null, skipped: [] },
+): ModelAvailabilityService {
+  const service = {
+    markTerminal: vi.fn(),
+    markHealthy: vi.fn(),
+    markRetryOncePerTurn: vi.fn(),
+    consumeStickyAttempt: vi.fn(),
+    snapshot: vi.fn(),
+    resetTurn: vi.fn(),
+    selectFirstAvailable: vi.fn().mockReturnValue(selection),
+  };
+
+  return service as unknown as ModelAvailabilityService;
+}