add map codec tests

mobile/lib/domain/models/value_codec.dart

@@ -78,7 +78,7 @@ final class MapCodec<K extends Object, V extends Object> extends ValueCodec<Map<
         final rawKey = entry.key;
         final rawValue = entry.value;
         if (rawKey is! String || rawValue is! String) {
-          return {};
+          continue;
         }
         final k = _keyCodec.decode(rawKey);
         final v = _valueCodec.decode(rawValue);

mobile/test/unit/utils/value_codec_test.dart

@@ -0,0 +1,99 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:immich_mobile/domain/models/value_codec.dart';
+
+enum _Fruit { apple, banana, cherry }
+
+void main() {
+  group('MapCodec', () {
+    group('encode', () {
+      test('serializes an empty map to an empty JSON object', () {
+        const codec = MapCodec<String, String>(PrimitiveCodec.string, PrimitiveCodec.string);
+        expect(codec.encode({}), '{}');
+      });
+
+      test('encodes a string-to-string map as a JSON object', () {
+        const codec = MapCodec<String, String>(PrimitiveCodec.string, PrimitiveCodec.string);
+        expect(codec.encode({'a': '1', 'b': '2'}), '{"a":"1","b":"2"}');
+      });
+
+      test('stringifies non-string values via the value codec', () {
+        const codec = MapCodec<String, int>(PrimitiveCodec.string, PrimitiveCodec.integer);
+        expect(codec.encode({'x': 10, 'y': 20}), '{"x":"10","y":"20"}');
+      });
+
+      test('stringifies non-string keys via the key codec', () {
+        const codec = MapCodec<int, String>(PrimitiveCodec.integer, PrimitiveCodec.string);
+        expect(codec.encode({1: 'one', 2: 'two'}), '{"1":"one","2":"two"}');
+      });
+    });
+
+    group('decode', () {
+      test('reconstructs a string-to-string map', () {
+        const codec = MapCodec<String, String>(PrimitiveCodec.string, PrimitiveCodec.string);
+        expect(codec.decode('{"a":"1","b":"2"}'), {'a': '1', 'b': '2'});
+      });
+
+      test('parses values back to their domain type', () {
+        const codec = MapCodec<String, int>(PrimitiveCodec.string, PrimitiveCodec.integer);
+        expect(codec.decode('{"x":"10","y":"20"}'), {'x': 10, 'y': 20});
+      });
+
+      test('parses keys back to their domain type', () {
+        const codec = MapCodec<int, String>(PrimitiveCodec.integer, PrimitiveCodec.string);
+        expect(codec.decode('{"1":"one","2":"two"}'), {1: 'one', 2: 'two'});
+      });
+
+      test('returns an empty map for an empty JSON object', () {
+        const codec = MapCodec<String, String>(PrimitiveCodec.string, PrimitiveCodec.string);
+        expect(codec.decode('{}'), isEmpty);
+      });
+
+      test('returns an empty map when the payload is not valid JSON', () {
+        const codec = MapCodec<String, String>(PrimitiveCodec.string, PrimitiveCodec.string);
+        expect(codec.decode('not json'), isEmpty);
+      });
+
+      test('returns an empty map when the JSON root is not an object', () {
+        const codec = MapCodec<String, String>(PrimitiveCodec.string, PrimitiveCodec.string);
+        expect(codec.decode('[]'), isEmpty);
+        expect(codec.decode('"a string"'), isEmpty);
+        expect(codec.decode('42'), isEmpty);
+      });
+
+      test('skips entries whose value is not a JSON string, keeping the rest', () {
+        const codec = MapCodec<String, int>(PrimitiveCodec.string, PrimitiveCodec.integer);
+        expect(codec.decode('{"x":1,"y":"20"}'), {'y': 20});
+      });
+
+      test('skips entries whose value is a nested object, keeping the rest', () {
+        const codec = MapCodec<String, String>(PrimitiveCodec.string, PrimitiveCodec.string);
+        expect(codec.decode('{"a":{"nested":"value"},"b":"ok"}'), {'b': 'ok'});
+      });
+
+      test('returns an empty map when every entry is malformed', () {
+        const codec = MapCodec<String, int>(PrimitiveCodec.string, PrimitiveCodec.integer);
+        expect(codec.decode('{"x":1,"y":2}'), isEmpty);
+      });
+    });
+
+    group('round trip', () {
+      test('preserves a primitive map through encode then decode', () {
+        const codec = MapCodec<String, int>(PrimitiveCodec.string, PrimitiveCodec.integer);
+        const original = {'one': 1, 'two': 2, 'three': 3};
+        expect(codec.decode(codec.encode(original)), original);
+      });
+
+      test('preserves an enum-valued map by composing with EnumCodec', () {
+        const codec = MapCodec<String, _Fruit>(PrimitiveCodec.string, EnumCodec(_Fruit.values));
+        const original = {'breakfast': _Fruit.banana, 'snack': _Fruit.apple};
+        expect(codec.decode(codec.encode(original)), original);
+      });
+
+      test('preserves a DateTime-valued map by composing with DateTimeCodec', () {
+        const codec = MapCodec<String, DateTime>(PrimitiveCodec.string, DateTimeCodec());
+        final original = {'created': DateTime.utc(2024, 1, 1, 12, 30)};
+        expect(codec.decode(codec.encode(original)), original);
+      });
+    });
+  });
+}