fix: use DATETIME_FORMAT_MS_WITH_TZ instead of isoformat for last_data_timestamp

Addresses review feedback: prefer sientia_do's explicit datetime format
constant over Python's isoformat()/fromisoformat() when round-tripping
last_data_timestamp through Redis, matching the convention already used
elsewhere in mongo_db.py and slot_manager.py.

orchestrator/activities/mongo_db.py

@@ -453,7 +453,7 @@ class MongoDB(SientiaMonitoring):
                 - metadata (dict[str, Any]): Workflow execution metadata
                 - collection_name (str): Name of the MongoDB collection
                 - last_data_timestamp (str | None): Last processed timestamp
-                  (ISO-format string) for filtering
+                  (formatted per ``DATETIME_FORMAT_MS_WITH_TZ``) for filtering
                 - base_data_filter (dict[str, Any]): Base query filter conditions
 
         Returns:
@@ -475,12 +475,15 @@ class MongoDB(SientiaMonitoring):
             else:
                 # ``notification_queue.timestamp`` is stored as a native BSON
                 # ``Date`` (``Notification.timestamp`` is a ``datetime``).
-                # ``last_data_timestamp`` arrives here as an ISO-format string
-                # (round-tripped through Redis), so it must be parsed back to
-                # ``datetime`` for the ``$gt`` comparison to be type-correct.
+                # ``last_data_timestamp`` arrives here as a string formatted per
+                # ``DATETIME_FORMAT_MS_WITH_TZ`` (round-tripped through Redis),
+                # so it must be parsed back to ``datetime`` for the ``$gt``
+                # comparison to be type-correct.
                 data_filter = {
                     **base_data_filter,
-                    'timestamp': {'$gt': datetime.fromisoformat(last_data_timestamp)},
+                    'timestamp': {
+                        '$gt': datetime.strptime(last_data_timestamp, DATETIME_FORMAT_MS_WITH_TZ)
+                    },
                 }
 
             self.debug(f'Data filter: {data_filter}', metadata=metadata)

orchestrator/activities/slot_manager.py

@@ -337,8 +337,9 @@ class SlotManager(SientiaMonitoring):
 
         # ``timestamp`` is a native ``datetime``/``Timestamp`` (BSON Date from
         # Mongo), which the Redis repository's plain ``json.dumps`` cannot
-        # serialize. Store it as an ISO-format string instead.
-        last_data_timestamp = last_data_timestamp.isoformat()
+        # serialize. Store it as a string formatted per
+        # ``DATETIME_FORMAT_MS_WITH_TZ`` instead.
+        last_data_timestamp = last_data_timestamp.strftime(DATETIME_FORMAT_MS_WITH_TZ)
 
         try:
             self.redis_repository.set(key, last_data_timestamp, ttl=60 * 60 * 5)