✅ Add JSONBodyHandler and tests

- Implemented JSONBodyHandler for automatic JSON body parsing.
- Added comprehensive unit tests for JSONBodyHandler covering successful decoding, invalid JSON, empty bodies, and error propagation.
- Refactored handler_test.go to improve test structure and resolve linting issues (funlen, gocritic).

handler.go

@@ -1,6 +1,7 @@
 package ezhandler
 
 import (
+	"encoding/json"
 	"io"
 	"net/http"
 )
@@ -42,3 +43,22 @@ func (fn ResponseHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) erro
 	_, err = io.Copy(w, body)
 	return err
 }
+
+// JSONBodyHandler can be used as a [Handler] which automatically parses the json body into a value, which is passed to the function.
+type JSONBodyHandler[V any] func(w http.ResponseWriter, r *http.Request, body V) error
+
+var _ Handler = JSONBodyHandler[map[string]any](nil)
+
+func (fn JSONBodyHandler[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
+	}
+
+	return fn(w, r, body)
+}