chore: add mypy cache to gitignore

Added .mypy_cache/ patterns to prevent committing mypy's type checking
cache directories. This keeps the repository clean and avoids conflicts
between different development environments.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: dguido

.gitignore

@@ -41,6 +41,10 @@ dist/
 pdm.lock
 .langchain.db
 
+# Mypy type checking cache
+.mypy_cache/
+**/.mypy_cache/
+
 *.bak
 *.orig