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
The italic regex \*([^*]+)\* used [^*] which matches newlines, causing
bullet lists with * markers to be incorrectly converted to italic text.
Changed to [^*\n]+ to prevent cross-line matching.
Adds 43 tests for _escape_mdv2 and format_message covering code blocks,
bold/italic, headers, links, mixed formatting, and the regression case.
