fix(backend): use timezone-aware UTC in memory modules (fix pytest DeprecationWarnings) (#1992)

* fix(backend): use timezone-aware UTC in memory modules

Replace datetime.utcnow() with datetime.now(timezone.utc) and a shared
utc_now_iso_z() helper so persisted ISO timestamps keep the trailing Z
suffix without triggering Python 3.12+ deprecation warnings.

Made-with: Cursor

* refactor(backend): use removesuffix for utc_now_iso_z suffix

Makes the +00:00 -> Z transform explicit for the trailing offset only
(Copilot review on PR #1992).

Made-with: Cursor

* style(backend): satisfy ruff UP017 with datetime.UTC in memory queue

Made-with: Cursor

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/packages/harness/deerflow/agents/memory/updater.py

@@ -5,14 +5,17 @@ import logging
 import math
 import re
 import uuid
-from datetime import datetime
 from typing import Any
 
 from deerflow.agents.memory.prompt import (
     MEMORY_UPDATE_PROMPT,
     format_conversation_for_update,
 )
-from deerflow.agents.memory.storage import create_empty_memory, get_memory_storage
+from deerflow.agents.memory.storage import (
+    create_empty_memory,
+    get_memory_storage,
+    utc_now_iso_z,
+)
 from deerflow.config.memory_config import get_memory_config
 from deerflow.models import create_chat_model
 
@@ -86,7 +89,7 @@ def create_memory_fact(
 
     normalized_category = category.strip() or "context"
     validated_confidence = _validate_confidence(confidence)
-    now = datetime.utcnow().isoformat() + "Z"
+    now = utc_now_iso_z()
     memory_data = get_memory_data(agent_name)
     updated_memory = dict(memory_data)
     facts = list(memory_data.get("facts", []))
@@ -376,7 +379,7 @@ class MemoryUpdater:
             Updated memory data.
         """
         config = get_memory_config()
-        now = datetime.utcnow().isoformat() + "Z"
+        now = utc_now_iso_z()
 
         # Update user sections
         user_updates = update_data.get("user", {})