fix: Parse JSON content in WebSocket message validator

**Motivations :**
- Fix WebSocket message validation that was rejecting valid handshake messages
- Allow content to be either object or JSON string as per protocol

**Modifications :**
- Updated validateMessageStructure to parse JSON string content
- Added proper error handling for invalid JSON strings
- Maintained backward compatibility with object content

**Pages affectées :**
- src/services/message-validator.ts - Enhanced content validation

src/services/message-validator.ts

@@ -106,9 +106,17 @@ export class MessageValidator {
       return { isValid: false, errors };
     }
 
-    // Vérifier le type du contenu
+    // Vérifier le type du contenu - peut être un objet ou une string JSON
-    if (typeof message.content !== 'object') {
+    if (typeof message.content === 'string') {
-      errors.push('Content must be an object');
+      // Parser le contenu JSON si c'est une string
+      try {
+        message.content = JSON.parse(message.content);
+      } catch (error) {
+        errors.push('Content must be valid JSON if it is a string');
+        return { isValid: false, errors };
+      }
+    } else if (typeof message.content !== 'object') {
+      errors.push('Content must be an object or valid JSON string');
       return { isValid: false, errors };
     }