fix: propagate memory tenant id in task collect (#15837)

### What problem does this PR solve?
Propagate `tenant_id` from memory task messages into task collection so
refactored task execution can build a valid context.

### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)

api/db/__init__.py

@@ -17,6 +17,8 @@
 from enum import IntEnum
 from enum import StrEnum
 
+from common.constants import PipelineTaskType
+
 
 class UserTenantRole(StrEnum):
     OWNER = 'owner'
@@ -59,14 +61,6 @@ class CanvasCategory(StrEnum):
     DataFlow = "dataflow_canvas"
 
 
-class PipelineTaskType(StrEnum):
-    PARSE = "Parse"
-    DOWNLOAD = "Download"
-    RAPTOR = "RAPTOR"
-    GRAPH_RAG = "GraphRAG"
-    MINDMAP = "Mindmap"
-
-
 VALID_PIPELINE_TASK_TYPES = {PipelineTaskType.PARSE, PipelineTaskType.DOWNLOAD, PipelineTaskType.RAPTOR, PipelineTaskType.GRAPH_RAG, PipelineTaskType.MINDMAP}