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/server_example_test.go

@@ -16,12 +16,12 @@ type SayHiParams struct {
 	Name string `json:"name"`
 }
 
-func SayHi(ctx context.Context, req *mcp.ServerRequest[*mcp.CallToolParamsFor[SayHiParams]]) (*mcp.CallToolResultFor[any], error) {
-	return &mcp.CallToolResultFor[any]{
+func SayHi(ctx context.Context, req *mcp.ServerRequest[*mcp.CallToolParams], args SayHiParams) (*mcp.CallToolResult, any, error) {
+	return &mcp.CallToolResult{
 		Content: []mcp.Content{
-			&mcp.TextContent{Text: "Hi " + req.Params.Arguments.Name},
+			&mcp.TextContent{Text: "Hi " + args.Name},
 		},
-	}, nil
+	}, nil, nil
 }
 
 func ExampleServer() {