mcp: change tool handler design (#325)

Change the design of tool handlers by removing genericity from the common path.
- `ToolHandler` gets the args as a `json.RawMessage`. Using Server.AddTool does no unmarshaling or schema validation.
- `ToolHandlerFor`, installed with `AddTool`, are the only generic pieces.

TODO:
uncomment and fix some tests

mcp/sse_example_test.go

@@ -18,12 +18,12 @@ type AddParams struct {
 	X, Y int
 }
 
-func Add(ctx context.Context, req *mcp.ServerRequest[*mcp.CallToolParamsFor[AddParams]]) (*mcp.CallToolResultFor[any], error) {
-	return &mcp.CallToolResultFor[any]{
+func Add(ctx context.Context, req *mcp.ServerRequest[*mcp.CallToolParams], args AddParams) (*mcp.CallToolResult, any, error) {
+	return &mcp.CallToolResult{
 		Content: []mcp.Content{
-			&mcp.TextContent{Text: fmt.Sprintf("%d", req.Params.Arguments.X+req.Params.Arguments.Y)},
+			&mcp.TextContent{Text: fmt.Sprintf("%d", args.X+args.Y)},
 		},
-	}, nil
+	}, nil, nil
 }
 
 func ExampleSSEHandler() {