🐛 fix: Resolve MeiliSearch Startup Sync Failure from Model Loading Order (#12397)

* fix: resolve MeiliSearch startup sync failure from model loading order

`indexSync.js` captures `mongoose.models.Message` and
`mongoose.models.Conversation` at module load time (lines 9-10).
Since PR #11830 moved model registration into `createModels()`,
these references are always `undefined` because `require('./indexSync')`
ran before `createModels(mongoose)` in `api/db/index.js`.

Move the `require('./indexSync')` below `createModels(mongoose)` so
Mongoose models are registered before `indexSync.js` captures them.

* fix: defer model lookups in indexSync, add load-order regression test

- Move mongoose.models.Message and mongoose.models.Conversation lookups
  from module-level into performSync() and the indexSync() catch block,
  eliminating the fragile load-order dependency that caused the original
  regression
- Add guard assertion that throws a clear error if models are missing
- Add comment in api/db/index.js documenting the createModels ordering
  constraint
- Add api/db/index.spec.js regression test to prevent future re-introduction
  of the load-order bug

* fix: strengthen regression test to verify call order, add guard in setTimeout path

- Rewrite index.spec.js to track call sequence via callOrder array,
  ensuring createModels executes before indexSync module loads
  (verified: test fails if order is reversed)
- Add missing model guard assertion in the setTimeout fallback path
  in indexSync.js for consistency with performSync

---------

Co-authored-by: Danny Avila <danny@librechat.ai>

api/db/index.spec.js

@@ -0,0 +1,26 @@
+describe('api/db/index.js', () => {
+  test('createModels is called before indexSync is loaded', () => {
+    jest.resetModules();
+
+    const callOrder = [];
+
+    jest.mock('@librechat/data-schemas', () => ({
+      createModels: jest.fn((m) => {
+        callOrder.push('createModels');
+        m.models.Message = { name: 'Message' };
+        m.models.Conversation = { name: 'Conversation' };
+      }),
+    }));
+
+    jest.mock('./indexSync', () => {
+      callOrder.push('indexSync');
+      return jest.fn();
+    });
+
+    jest.mock('./connect', () => ({ connectDb: jest.fn() }));
+
+    require('./index');
+
+    expect(callOrder).toEqual(['createModels', 'indexSync']);
+  });
+});