Add markdown-it-py and mdit-py-plugins to poetry.lock

Minimal update starting from upstream/main's lockfile:
- markdown-it-py 4.0.0 (mcp-only) → 3.0.0 (main + mcp), required by ^3.0 constraint
- mdit-py-plugins 0.4.2 added to main group (new direct dependency)
- mdurl groups updated to include main (transitive dep of markdown-it-py)
- content-hash refreshed; dev-group annotations corrected to match pyproject.toml

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: MattFisher