feat(backup): expose incremental_base_backup_id on BackupListReturn

GET /backups/{backend} returns incremental_base_backup_id per item (see
BackupListResponseItems0 and Scheduler.List in weaviate/weaviate), but
BackupListReturn does not declare the field, so list_backups() silently
drops it and callers cannot tell which base backup an incremental backup
was built on without a separate get_create_status() call per backup.

Add the field as Optional[str] defaulting to None: the server omits it for
non-incremental backups and only populates it for callers it has confirmed
as root, so its absence must stay non-fatal.

Author: Anai-Guo

test/test_backup.py

@@ -0,0 +1,41 @@
+from typing import Any, Dict
+
+import pytest
+
+from weaviate.backup.backup import BackupListReturn, BackupStatus
+
+
+def _list_entry(**overrides: Any) -> Dict[str, Any]:
+    """A single item as Weaviate's ``GET /backups/{backend}`` returns it."""
+    entry: Dict[str, Any] = {
+        "id": "my-backup",
+        "classes": ["Article"],
+        "status": "SUCCESS",
+        "startedAt": "2026-08-01T10:00:00.000Z",
+        "completedAt": "2026-08-01T10:05:00.000Z",
+        "size": 1.5,
+    }
+    entry.update(overrides)
+    return entry
+
+
+def test_list_return_exposes_incremental_base_backup_id() -> None:
+    backup = BackupListReturn(**_list_entry(incremental_base_backup_id="base-backup"))
+
+    assert backup.incremental_base_backup_id == "base-backup"
+    assert backup.backup_id == "my-backup"
+    assert backup.collections == ["Article"]
+    assert backup.status == BackupStatus.SUCCESS
+
+
+@pytest.mark.parametrize(
+    "entry",
+    [
+        pytest.param(_list_entry(), id="field_omitted"),
+        pytest.param(_list_entry(incremental_base_backup_id=None), id="field_null"),
+    ],
+)
+def test_list_return_defaults_incremental_base_backup_id_to_none(entry: Dict[str, Any]) -> None:
+    # The server omits the field for non-incremental backups, and only populates
+    # it for callers it has confirmed as root, so absent must not be an error.
+    assert BackupListReturn(**entry).incremental_base_backup_id is None

weaviate/backup/backup.py

@@ -104,3 +104,6 @@ class BackupListReturn(BaseModel):
     started_at: Optional[datetime] = Field(alias="startedAt", default=None)
     completed_at: Optional[datetime] = Field(alias="completedAt", default=None)
     size: float = Field(default=0)
+    # None when the backup is not incremental, and also when the server omits the
+    # field: it is only returned to callers Weaviate has confirmed as root.
+    incremental_base_backup_id: Optional[str] = Field(default=None)