fix(core): reject duplicate mock generator types during normalization

Validates that each mock type (msw, faker) appears at most once in
mock.generators. Duplicate entries would produce conflicting file
outputs, so we fail fast with a clear error message.

Also removes an unnecessary nullish coalescing on the required
generators field (lint: @typescript-eslint/no-unnecessary-condition).

Author: jakiestfu

packages/orval/src/utils/options.ts

@@ -190,7 +190,7 @@ export async function normalizeOptions(
   } else if (mocksOption && typeof mocksOption === 'object') {
     mocks = {
       indexMockFiles: mocksOption.indexMockFiles ?? false,
-      generators: (mocksOption.generators ?? []).map((m) =>
+      generators: mocksOption.generators.map((m) =>
         isFunction(m)
           ? m
           : ({
@@ -201,6 +201,17 @@ export async function normalizeOptions(
     };
   }
 
+  const seenMockTypes = new Set<string>();
+  for (const entry of mocks.generators) {
+    if (isFunction(entry)) continue;
+    if (seenMockTypes.has(entry.type)) {
+      throw new Error(
+        `Duplicate mock generator type "${entry.type}". Each type can only appear once in mock.generators.`,
+      );
+    }
+    seenMockTypes.add(entry.type);
+  }
+
   const defaultFileExtension = '.ts';
 
   // `useQuery` / `useMutation` defaults are applied per-verb in