feat(sync): Update xAI collection file addition endpoint and improve documentation

services/xai_service.py

@@ -112,12 +112,9 @@ class XAIService:
 
     async def add_to_collection(self, collection_id: str, file_id: str) -> None:
         """
-        Fügt eine Datei einer xAI-Collection (Vector Store) hinzu.
+        Fügt eine Datei einer xAI-Collection hinzu.
 
-        POST https://api.x.ai/v1/vector_stores/{vector_store_id}/files
-
-        Uses the OpenAI-compatible API pattern for adding files to vector stores.
-        This triggers proper indexing and processing.
+        POST https://management-api.x.ai/v1/collections/{collection_id}/documents/{file_id}
 
         Raises:
             RuntimeError: bei HTTP-Fehler
@@ -125,16 +122,10 @@ class XAIService:
         self._log(f"📚 Adding file {file_id} to collection {collection_id}")
 
         session = await self._get_session()
-        # Use the OpenAI-compatible endpoint (not management API)
-        url = f"{XAI_FILES_URL}/v1/vector_stores/{collection_id}/files"
-        headers = {
-            "Authorization": f"Bearer {self.api_key}",
-            "Content-Type": "application/json",
-        }
+        url = f"{XAI_MANAGEMENT_URL}/v1/collections/{collection_id}/documents/{file_id}"
+        headers = {"Authorization": f"Bearer {self.management_key}"}
 
-        payload = {"file_id": file_id}
-
-        async with session.post(url, json=payload, headers=headers) as response:
+        async with session.post(url, headers=headers) as response:
             if response.status not in (200, 201):
                 raw = await response.text()
                 raise RuntimeError(