fix(api): improve error handling for invalid JSON body

Adds a specific error handler for `CheckedFromJsonException` to return a
400 Bad Request instead of a generic 500 Internal Server Error when
deserialization fails due to invalid client input.

This provides more accurate feedback to API clients when they send a
request body that is missing required fields or contains incorrect
data types.

Author: fulleni

lib/src/middlewares/error_handler.dart

@@ -5,6 +5,7 @@ import 'dart:io';
 
 import 'package:dart_frog/dart_frog.dart';
 import 'package:ht_shared/ht_shared.dart';
+import 'package:json_annotation/json_annotation.dart';
 
 /// Middleware that catches errors and converts them into
 /// standardized JSON responses.
@@ -26,6 +27,21 @@ Middleware errorHandler() {
             'error': {'code': errorCode, 'message': e.message},
           },
         );
+      } on CheckedFromJsonException catch (e, stackTrace) {
+        // Handle json_serializable validation errors. These are client errors.
+        final field = e.key ?? 'unknown';
+        final message = 'Invalid request body: Field "$field" has an '
+            'invalid value or is missing. ${e.message}';
+        print('CheckedFromJsonException Caught: $e\n$stackTrace');
+        return Response.json(
+          statusCode: HttpStatus.badRequest, // 400
+          body: {
+            'error': {
+              'code': 'invalidField',
+              'message': message,
+            },
+          },
+        );
       } on FormatException catch (e, stackTrace) {
         // Handle data format/parsing errors (often indicates bad client input)
         print('FormatException Caught: $e\n$stackTrace'); // Log for debugging

pubspec.yaml

@@ -34,7 +34,7 @@ dependencies:
   ht_shared:
     git:
       url: https://github.com/headlines-toolkit/ht-shared.git
-
+  json_annotation: ^4.9.0
   logging: ^1.3.0
   meta: ^1.16.0
   mongo_dart: ^0.10.5