Add unit testing for map serialization

packages/langchain_core/test/utils/map_serialization.dart

@@ -0,0 +1,52 @@
+import 'dart:convert';
+
+import 'package:langchain_core/chat_models.dart';
+import 'package:langchain_core/prompts.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('Map Serialization', () {
+    /// Test messages for testing serialization. Designed to cover every possible novel property in the models.
+    final testMessages = <ChatMessage>[
+      ChatMessage.system('This is a system prompt'),
+      ChatMessage.human(
+        ChatMessageContent.multiModal([
+          // Tests all message content parts
+          ChatMessageContent.text('Basic Text'),
+          ChatMessageContent.image(
+            data: 'not-real-image-data',
+            imageDetail: ChatMessageContentImageDetail.low,
+            mimeType: 'any/fake',
+          ),
+        ]),
+      ),
+      ChatMessage.ai(
+        'Assistant Response',
+        toolCalls: const [
+          AIChatMessageToolCall(
+            id: 'some-id',
+            name: 'some name',
+            argumentsRaw: '{"a": 1, "b": 2}',
+            arguments: {'a': 1, 'b': 2},
+          ),
+          AIChatMessageToolCall(
+            id: 'some-id2',
+            name: 'some name',
+            argumentsRaw: '{"c": 3, "d": 4}',
+            arguments: {'c': 3, 'd': 4},
+          )
+        ],
+      ),
+      ChatMessage.tool(toolCallId: 'some-id2', content: 'Tool Call content'),
+      ChatMessage.custom('huh?', role: 'internal-monologue'),
+    ];
+
+    test('Prompt Values', () async {
+      final promptValue = PromptValue.chat(testMessages);
+
+      // Basic encode/decode re-encode check
+      expect(jsonEncode(PromptValue.fromMap(promptValue.toMap()).toMap()),
+          jsonEncode(promptValue.toMap()));
+    });
+  });
+}