fix: satisfy mypy in put_last_data_timestamp after utc=True normalization

CI mypy caught what my local environment couldn't (Python 3.14 has no
compatible numpy stub for the project's python_version=3.11 mypy target,
so mypy couldn't run locally at all):

  slot_manager.py:353: error: Incompatible types in assignment
    (expression has type "str", variable has type "Timestamp")
  slot_manager.py:368: error: Incompatible return value type
    (got "Timestamp", expected "str | None")

Reassigning `last_data_timestamp` from a Timestamp to its strftime() string
trips mypy's inference (it fixes an unannotated local's type from its first
assignment). Split into a separate `last_data_timestamp_str` for the string
form instead of reusing the name.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

orchestrator/activities/slot_manager.py

@@ -349,11 +349,15 @@ class SlotManager(SientiaMonitoring):
 
         # The Redis repository's plain ``json.dumps`` cannot serialize a
         # ``Timestamp``/``datetime`` directly, so 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)
+        # formatted per ``DATETIME_FORMAT_MS_WITH_TZ`` instead. Kept as a
+        # separate variable (rather than reassigning ``last_data_timestamp``)
+        # since it's a different type — mypy infers a local's type from its
+        # first assignment and flags a same-name reassignment to an
+        # incompatible type as an error.
+        last_data_timestamp_str = last_data_timestamp.strftime(DATETIME_FORMAT_MS_WITH_TZ)
 
         try:
-            self.redis_repository.set(key, last_data_timestamp, ttl=60 * 60 * 5)
+            self.redis_repository.set(key, last_data_timestamp_str, ttl=60 * 60 * 5)
         except Exception as e:
             self.send_notification(
                 metadata=metadata,
@@ -365,7 +369,7 @@ class SlotManager(SientiaMonitoring):
             )
             raise e
 
-        return last_data_timestamp
+        return last_data_timestamp_str
 
     @activity.defn(name='filter_notification_alerts')
     def filter_notification_alerts(self, input_data: dict[str, Any]) -> dict[str, Any]: