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

internal/readme/server/server.go

@@ -16,10 +16,10 @@ type HiParams struct {
 	Name string `json:"name" jsonschema:"the name of the person to greet"`
 }
 
-func SayHi(ctx context.Context, req *mcp.ServerRequest[*mcp.CallToolParamsFor[HiParams]]) (*mcp.CallToolResultFor[any], error) {
-	return &mcp.CallToolResultFor[any]{
-		Content: []mcp.Content{&mcp.TextContent{Text: "Hi " + req.Params.Arguments.Name}},
-	}, nil
+func SayHi(ctx context.Context, req *mcp.ServerRequest[*mcp.CallToolParams], args HiParams) (*mcp.CallToolResult, any, error) {
+	return &mcp.CallToolResult{
+		Content: []mcp.Content{&mcp.TextContent{Text: "Hi " + args.Name}},
+	}, nil, nil
 }
 
 func main() {