🏺 fix: Restore Archive Functionality with Dedicated Endpoint (#11183)

The archive conversation feature was broken after the `/api/convos/update`
route was modified to only handle title updates. The frontend was sending
`{ conversationId, isArchived }` to the update endpoint, but the backend
was only extracting `title` and ignoring the `isArchived` field entirely.

This fix implements a dedicated `/api/convos/archive` endpoint to restore
the archive/unarchive functionality.

Changes:

packages/data-provider/src/api-endpoints.ts:
- Add `archiveConversation()` endpoint returning `/api/convos/archive`

packages/data-provider/src/data-service.ts:
- Update `archiveConversation()` to use dedicated archive endpoint

api/server/routes/convos.js:
- Add `POST /archive` route with validation for `conversationId` (required)
  and `isArchived` (must be boolean)

api/server/routes/__tests__/convos.spec.js:
- Add test coverage for archive endpoint (success, validation, error cases)

api/server/routes/convos.js

@@ -152,6 +152,37 @@ router.delete('/all', async (req, res) => {
   }
 });
 
+/**
+ * Archives or unarchives a conversation.
+ * @route POST /archive
+ * @param {string} req.body.arg.conversationId - The conversation ID to archive/unarchive.
+ * @param {boolean} req.body.arg.isArchived - Whether to archive (true) or unarchive (false).
+ * @returns {object} 200 - The updated conversation object.
+ */
+router.post('/archive', validateConvoAccess, async (req, res) => {
+  const { conversationId, isArchived } = req.body.arg ?? {};
+
+  if (!conversationId) {
+    return res.status(400).json({ error: 'conversationId is required' });
+  }
+
+  if (typeof isArchived !== 'boolean') {
+    return res.status(400).json({ error: 'isArchived must be a boolean' });
+  }
+
+  try {
+    const dbResponse = await saveConvo(
+      req,
+      { conversationId, isArchived },
+      { context: `POST /api/convos/archive ${conversationId}` },
+    );
+    res.status(200).json(dbResponse);
+  } catch (error) {
+    logger.error('Error archiving conversation', error);
+    res.status(500).send('Error archiving conversation');
+  }
+});
+
 /** Maximum allowed length for conversation titles */
 const MAX_CONVO_TITLE_LENGTH = 1024;