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

packages/core/src/availability/errorClassification.ts

@@ -0,0 +1,25 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import {
+  TerminalQuotaError,
+  RetryableQuotaError,
+} from '../utils/googleQuotaErrors.js';
+import { ModelNotFoundError } from '../utils/httpErrors.js';
+import type { FailureKind } from './modelPolicy.js';
+
+export function classifyFailureKind(error: unknown): FailureKind {
+  if (error instanceof TerminalQuotaError) {
+    return 'terminal';
+  }
+  if (error instanceof RetryableQuotaError) {
+    return 'transient';
+  }
+  if (error instanceof ModelNotFoundError) {
+    return 'not_found';
+  }
+  return 'unknown';
+}