Refactor MCPToolRegistry class side methods: xxxForTool:

[NicolasAnquetil]

all static methods xxxForTool: would seem better implemented on the tools themselves.

mutabilityForTool: and riskForTool: seem good candidates to start with

[Gabriel-Darbord]

Completed by #108: tool metadata is now owned by tools, and MCPToolRegistry has no class-side responsibilities.