Discoverable tools

Implement tools/list with two descriptors ordered by name: add and echo. Copy the names, descriptions, and schemas shown by the tests exactly; a schema is part of the public interface.

tools/call receives params.name and params.arguments. add accepts two numbers and represents their sum as JSON text without unnecessary zeros. echo accepts a string. Both return content with one text block and isError:false.

Do not confuse protocol errors with tool errors. In the final stage, invalid arguments will become a tool result with isError:true.

• The list and its properties have stable order.
• Both tools produce MCP content.
• Previous stages remain supported.