Previously, PTB is producing incorrect results when formatting a message into the Markdown v2 format (Message.(text|caption)_markdown_v2). Specifically, when there is any of the need-to-escape characters _*~`>#+-=|{}.! appear in a nested entities, the last few characters was repeated at the end of the range.

For example

* <hashtag /> is used here just to represent the hashtag entity, Telegram does not support such a tag.

The issue is suspected to be caused by the used of escaped text sequence for nested entities, which is causing a discrepancy in the length of text due to the introduction of \.

This PR fixes the issue by using the original text (orig_text) as a basis for nested entity parsing. It also includes updated tests to address this issue.