Fixes #2210
This pull request includes changes to the remove_mention_text method in turn_context.py and adds a new test case in test_turn_context.py to ensure the method works correctly with custom mentions.

Improvements to remove_mention_text method:

• libraries/botbuilder-core/botbuilder/core/turn_context.py: Modified the remove_mention_text method to handle cases where the mention text is not directly available by using either the text or name property from additional_properties.

Additions to test cases:

• libraries/botbuilder-core/tests/test_turn_context.py: Added a new test case test_should_remove_custom_mention_from_activity to verify that the remove_mention_text method correctly removes custom mentions from the activity text.