Normalized runtime descriptor for one registered tool.
Every tool module — classic use Noizu.MCP.Server.Tool modules and
use Noizu.MCP.Server.Toolkit multi-tool modules alike — exports
__mcp_tools__/0 returning a list of these. The server runtime
(Noizu.MCP.Server.Features.Tools.expand/1) works exclusively on specs:
the definition is what tools/list advertises, module/fun/arity
are how tools/call invokes the handler, and cast_plan/output_schema
drive argument casting and structured-output checking.