fix: Make AgentDto configuration fields nullable for list endpoint compatibility

The backend /api/agents list endpoint returns a lightweight DTO without
configuration fields (temperature, maxTokens, systemPrompt, enableMemory,
conversationWindowSize). This caused a TypeError when parsing the response
as these fields were required in AgentDto.

Changes:
- Made 5 configuration fields nullable in AgentDto
- Updated constructor to accept optional values
- Fixed fromJson() to safely handle null values with explicit checks
- Maintains backward compatibility with full agent detail responses

This fix resolves the "Error Loading Agents" issue and allows the agents
page to display correctly. List endpoint now parses successfully while
detail endpoints still provide full configuration.

Fixes: TypeError: null: type 'Null' is not a subtype of type 'num'

Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

FRONTEND/lib/api/endpoints/agent_endpoint.dart

@@ -243,11 +243,11 @@ class AgentDto {
   final String modelName;
   final ModelProviderType providerType;
   final String? modelEndpoint;
-  final double temperature;
-  final int maxTokens;
-  final String systemPrompt;
-  final bool enableMemory;
-  final int conversationWindowSize;
+  final double? temperature;
+  final int? maxTokens;
+  final String? systemPrompt;
+  final bool? enableMemory;
+  final int? conversationWindowSize;
   final AgentStatus status;
   final DateTime createdAt;
   final DateTime updatedAt;
@@ -261,11 +261,11 @@ class AgentDto {
     required this.modelName,
     required this.providerType,
     this.modelEndpoint,
-    required this.temperature,
-    required this.maxTokens,
-    required this.systemPrompt,
-    required this.enableMemory,
-    required this.conversationWindowSize,
+    this.temperature,
+    this.maxTokens,
+    this.systemPrompt,
+    this.enableMemory,
+    this.conversationWindowSize,
     required this.status,
     required this.createdAt,
     required this.updatedAt,
@@ -300,11 +300,11 @@ class AgentDto {
       modelName: json['modelName'] as String,
       providerType: parseProviderType(json['providerType']),
       modelEndpoint: json['modelEndpoint'] as String?,
-      temperature: (json['temperature'] as num).toDouble(),
-      maxTokens: json['maxTokens'] as int,
-      systemPrompt: json['systemPrompt'] as String,
-      enableMemory: json['enableMemory'] as bool,
-      conversationWindowSize: json['conversationWindowSize'] as int,
+      temperature: json['temperature'] != null ? (json['temperature'] as num).toDouble() : null,
+      maxTokens: json['maxTokens'] as int?,
+      systemPrompt: json['systemPrompt'] as String?,
+      enableMemory: json['enableMemory'] as bool?,
+      conversationWindowSize: json['conversationWindowSize'] as int?,
       status: parseAgentStatus(json['status']),
       createdAt: DateTime.parse(json['createdAt'] as String),
       updatedAt: DateTime.parse(json['updatedAt'] as String),