fix(search): tighten mock type cast for better type safety

Replace `as any` cast with `as unknown as ReturnType<typeof createMeiliClient>`
in the mock client factory. This preserves type safety without requiring the mock
to implement the full SDK interface.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/search/src/index.test.ts

@@ -4,6 +4,7 @@ import {
   indexMessage,
   deleteMessage,
   MeiliDocument,
+  createMeiliClient,
 } from './index';
 
 function makeMockClient() {
@@ -15,7 +16,7 @@ function makeMockClient() {
     addDocuments: mockAddDocuments,
     deleteDocument: mockDeleteDocument,
   });
-  return { client: { index: mockIndex } as any, mockIndex, mockUpdateSettings, mockAddDocuments, mockDeleteDocument };
+  return { client: { index: mockIndex } as unknown as ReturnType<typeof createMeiliClient>, mockIndex, mockUpdateSettings, mockAddDocuments, mockDeleteDocument };
 }
 
 describe('MESSAGES_INDEX', () => {