Surface real Drive API errors instead of silently reporting fake success

list_folder_files_recursive logged a warning and returned an empty list on
any files.list failure (e.g. Drive API not enabled, revoked access), which
made reindex_drive report "success, 0 files" — indistinguishable from an
actually-empty folder. It now raises with Google's own error message (often
directly actionable, e.g. a link to enable the API), which reindex_drive's
existing error handling surfaces as an "error" status with that message in
the Chat Agent status log.

Found via a real 403 on ezofis.com's Space (Drive API not yet enabled on
the Google Cloud project); verified by reproducing that exact response.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/chat_agent_drive.py

@@ -154,8 +154,22 @@ def refresh_access_token(refresh_token: str) -> str:
     return token
 
 
+def _drive_api_error_message(resp: requests.Response) -> str:
+    """Google's error body is usually {"error": {"message": "..."}} — surface that verbatim
+    (it's often directly actionable, e.g. a link to enable the API) rather than raw JSON."""
+    try:
+        msg = resp.json().get("error", {}).get("message")
+        if msg:
+            return str(msg)
+    except Exception:
+        pass
+    return resp.text[:300]
+
+
 def list_folder_files_recursive(access_token: str, folder_id: str) -> List[Dict[str, Any]]:
-    """BFS through a Drive folder (and subfolders), returning flat list of non-folder file metadata."""
+    """BFS through a Drive folder (and subfolders), returning flat list of non-folder file metadata.
+    Raises RuntimeError with Google's own message on any listing failure (e.g. Drive API disabled,
+    revoked access) — a caller silently treating that as "0 files" would misreport it as success."""
     headers = {"Authorization": f"Bearer {access_token}"}
     out: List[Dict[str, Any]] = []
     queue = [folder_id]
@@ -176,8 +190,9 @@ def list_folder_files_recursive(access_token: str, folder_id: str) -> List[Dict[
                 params["pageToken"] = page_token
             r = requests.get(f"{DRIVE_API_BASE}/files", headers=headers, params=params, timeout=30)
             if r.status_code != 200:
-                logger.warning("Drive files.list failed: %s", r.text[:300])
-                break
+                msg = _drive_api_error_message(r)
+                logger.warning("Drive files.list failed (%s): %s", r.status_code, msg)
+                raise RuntimeError(f"Drive API error ({r.status_code}): {msg}")
             data = r.json()
             for f in data.get("files", []):
                 if f.get("mimeType") == GOOGLE_FOLDER_MIME: