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/queue.py

@@ -4,7 +4,7 @@ import logging
 import threading
 import time
 from dataclasses import dataclass, field
-from datetime import datetime
+from datetime import UTC, datetime
 from typing import Any
 
 from deerflow.config.memory_config import get_memory_config
@@ -18,7 +18,7 @@ class ConversationContext:
 
     thread_id: str
     messages: list[Any]
-    timestamp: datetime = field(default_factory=datetime.utcnow)
+    timestamp: datetime = field(default_factory=lambda: datetime.now(UTC))
     agent_name: str | None = None
     correction_detected: bool = False
     reinforcement_detected: bool = False