feat: implement parallel file downloading and indexing in Google Drive indexer

- Added `_download_files_parallel` function to enable concurrent downloading of files from Google Drive, improving efficiency in document processing.
- Introduced `_download_and_index` function to handle the parallel downloading and indexing phases, streamlining the overall workflow.
- Updated `_index_full_scan` and `_index_with_delta_sync` methods to utilize the new parallel downloading functionality, enhancing performance.
- Added unit tests to validate the new parallel downloading and indexing logic, ensuring robustness and error handling during document processing.

surfsense_backend/tests/unit/connector_indexers/conftest.py

@@ -0,0 +1,34 @@
+"""Pre-register the connector_indexers package to bypass a circular import
+in its ``__init__.py`` (airtable_indexer -> routes -> connector_indexers).
+
+This lets tests import individual indexer modules (e.g.
+``google_drive_indexer``) without triggering the full package init.
+"""
+
+import sys
+import types
+from pathlib import Path
+
+_BACKEND = Path(__file__).resolve().parents[3]
+
+
+def _stub_package(dotted: str, fs_dir: Path) -> None:
+    if dotted not in sys.modules:
+        mod = types.ModuleType(dotted)
+        mod.__path__ = [str(fs_dir)]
+        mod.__package__ = dotted
+        sys.modules[dotted] = mod
+
+    parts = dotted.split(".")
+    if len(parts) > 1:
+        parent_dotted = ".".join(parts[:-1])
+        parent = sys.modules.get(parent_dotted)
+        if parent is not None:
+            setattr(parent, parts[-1], sys.modules[dotted])
+
+
+_stub_package("app.tasks", _BACKEND / "app" / "tasks")
+_stub_package(
+    "app.tasks.connector_indexers",
+    _BACKEND / "app" / "tasks" / "connector_indexers",
+)