Langchain-mcp-tools

Added
- `make test-publish` target to handle publication more carefully

Changed
- Estimate the size of returning text in a simpler way
- Return a text with reasonable explanation when no text return found

Fixed
- [Issue 8](https://github.com/hideya/langchain-mcp-tools-py/issues/8): Content field of tool result is wrong
- Better checks when converting MCP's results into `str`

Changed
- Update example code in README.md to use `claude-3-5-sonnet-latest`
instead of `haiku` which is sometimes less capable to handle results from MCP