fix: strip MarkdownV2 italic markers in Telegram plaintext fallback

When MarkdownV2 parsing fails, _strip_mdv2() removes escape backslashes
and bold markers (*text*) but missed italic markers (_text_). Users saw
raw underscores around italic text in the plaintext fallback.

- Add regex to strip _text_ italic markers in _strip_mdv2()
- Use word boundary lookaround to preserve snake_case identifiers
- Add tests for _strip_mdv2 covering italic, bold, snake_case, and edge cases

gateway/platforms/telegram.py

@@ -86,6 +86,9 @@ def _strip_mdv2(text: str) -> str:
     cleaned = re.sub(r'\\([_*\[\]()~`>#\+\-=|{}.!\\])', r'\1', text)
     # Remove MarkdownV2 bold markers that format_message converted from **bold**
     cleaned = re.sub(r'\*([^*]+)\*', r'\1', cleaned)
+    # Remove MarkdownV2 italic markers that format_message converted from *italic*
+    # Use word boundary (\b) to avoid breaking snake_case like my_variable_name
+    cleaned = re.sub(r'(?<!\w)_([^_]+)_(?!\w)', r'\1', cleaned)
     return cleaned