Gateway tools for lazy tool discovery and execution.

Uses callbacks on ctx for tool gateway operations:

• ctx.callbacks[:search_tools] - (query, opts) -> [result]

• ctx.callbacks[:get_tool_schema] - (name) -> {:ok, schema} | {:error, reason}

• ctx.callbacks[:execute_external_tool] - (name, args, ctx) -> {:ok, result} | {:error, reason}