fix: optional markdown_text for append

Author: zimeg

slack_sdk/web/async_client.py

@@ -2625,7 +2625,7 @@ async def chat_appendStream(
         *,
         channel: str,
         ts: str,
-        markdown_text: Optional[str],
+        markdown_text: Optional[str] = None,
         **kwargs,
     ) -> AsyncSlackResponse:
         """Appends text to an existing streaming conversation.

slack_sdk/web/client.py

@@ -2615,7 +2615,7 @@ def chat_appendStream(
         *,
         channel: str,
         ts: str,
-        markdown_text: Optional[str],
+        markdown_text: Optional[str] = None,
         **kwargs,
     ) -> SlackResponse:
         """Appends text to an existing streaming conversation.

slack_sdk/web/legacy_client.py

@@ -2627,7 +2627,7 @@ def chat_appendStream(
         *,
         channel: str,
         ts: str,
-        markdown_text: Optional[str],
+        markdown_text: Optional[str] = None,
         **kwargs,
     ) -> Union[Future, SlackResponse]:
         """Appends text to an existing streaming conversation.