Add unit tests for turn_context (#65)

* port test case from botframework for turn-context

* fix  model to derive from  instead of

* format tests with black

* fix branches

* change entity type to mention subtype

* Revert turn_context to original

* set default value for text in case that doesn't exist in a mention

* all test_turn_context tests passing

---------

Co-authored-by: = <=>
Co-authored-by: Axel Suárez <axsuarez@microsoft.com>

Author: kylerohnmsft

libraries/Builder/microsoft-agents-builder/microsoft/agents/builder/turn_context.py

@@ -400,7 +400,7 @@ def remove_mention_text(activity: Activity, identifier: str) -> str:
             if mention.additional_properties["mentioned"]["id"] == identifier:
                 mention_name_match = re.match(
                     r"<at(.*)>(.*?)<\/at>",
-                    re.escape(mention.additional_properties["text"]),
+                    re.escape(mention.additional_properties.get("text", "")),
                     re.IGNORECASE,
                 )
                 if mention_name_match: