chore: fix type hints in thread decorator

Signed-off-by: Daniel Bluhm <dbluhm@pm.me>

aries_cloudagent/messaging/agent_message.py

@@ -305,7 +305,7 @@ def _thread(self) -> ThreadDecorator:
         return self._decorators.get("thread")
 
     @_thread.setter
-    def _thread(self, val: Union[ThreadDecorator, dict]):
+    def _thread(self, val: Union[ThreadDecorator, dict, None]):
         """Setter for the message's thread decorator.
 
         Args:
@@ -335,7 +335,7 @@ def assign_thread_from(self, msg: "AgentMessage"):
             pthid = thread and thread.pthid
             self.assign_thread_id(thid, pthid)
 
-    def assign_thread_id(self, thid: str, pthid: str = None):
+    def assign_thread_id(self, thid: str, pthid: Optional[str] = None):
         """Assign a specific thread ID.
 
         Args:

aries_cloudagent/messaging/decorators/thread_decorator.py

@@ -4,7 +4,7 @@
 context from previous messages.
 """
 
-from typing import Mapping
+from typing import Mapping, Optional
 
 from marshmallow import EXCLUDE, fields
 
@@ -23,10 +23,10 @@ class Meta:
     def __init__(
         self,
         *,
-        thid: str = None,
-        pthid: str = None,
-        sender_order: int = None,
-        received_orders: Mapping = None,
+        thid: Optional[str] = None,
+        pthid: Optional[str] = None,
+        sender_order: Optional[int] = None,
+        received_orders: Optional[Mapping] = None,
     ):
         """Initialize a ThreadDecorator instance.