✨ Add JSONResponder type and tests

- Introduced JSONResponder type in handler.go, combining JSON body parsing with ResponseHelper handling.
- Implemented ServeHTTP for JSONResponder to automatically decode JSON requests and return structured responses.
- Added comprehensive unit tests for JSONResponder in handler_test.go, covering successful responses, invalid JSON, empty bodies, and error propagation.
- Ensured all new tests adhere to existing project conventions and pass linting checks.

handler.go

@@ -62,3 +62,27 @@ func (fn JSONBodyHandler[V]) ServeHTTP(w http.ResponseWriter, r *http.Request) e
 
 	return fn(w, r, body)
 }
+
+// JSONResponder can be used as a [Handler] which automatically parses the json body into a value, which is passed to the function.
+// It also returns a [ResponseHelper], instead of passing an [http.ResponseWriter].
+type JSONResponder[V any] func(r *http.Request, body V) (ResponseHelper, error)
+
+var _ Handler = JSONResponder[map[string]any](nil)
+
+func (fn JSONResponder[V]) ServeHTTP(w http.ResponseWriter, r *http.Request) error {
+	reqBody := r.Body
+	//nolint:errcheck // This is usually fine
+	defer reqBody.Close()
+
+	dec := json.NewDecoder(reqBody)
+	var body V
+	if err := dec.Decode(&body); err != nil {
+		return err
+	}
+
+	var rh ResponseHandler = func(req *http.Request) (ResponseHelper, error) {
+		return fn(req, body)
+	}
+
+	return rh.ServeHTTP(w, r)
+}