Handle agent_tool_request event type in MessageHandler

When the ElevenLabs agent calls a server-side tool (webhook), the SDK
receives an `agent_tool_request` event. Previously this hit the default
branch, logging "Unknown event type: agent_tool_request".

This adds proper handling mirroring the existing `agent_tool_response`
pattern:
- `AgentToolRequest` event model with tool_name, tool_call_id,
  tool_type, and event_id (matching the AsyncAPI schema in
  elevenlabs/packages)
- `onAgentToolRequest` callback in `ConversationCallbacks`
- Handler in `MessageHandler` to parse and dispatch the event

This allows SDK consumers to react to pending tool calls, e.g. showing
a loading indicator while a webhook executes.

Author: seahyc

lib/src/messaging/message_handler.dart

@@ -96,6 +96,11 @@ class MessageHandler {
           _handleMcpConnectionStatus(json);
           break;
 
+        case 'agent_tool_request':
+          callbacks.onDebug?.call(json);
+          _handleAgentToolRequest(json);
+          break;
+
         case 'agent_tool_response':
           callbacks.onDebug?.call(json);
           _handleAgentToolResponse(json);
@@ -251,6 +256,11 @@ class MessageHandler {
     callbacks.onMcpConnectionStatus?.call(status);
   }
 
+  void _handleAgentToolRequest(Map<String, dynamic> json) {
+    final request = AgentToolRequest.fromJson(json);
+    callbacks.onAgentToolRequest?.call(request);
+  }
+
   void _handleAgentToolResponse(Map<String, dynamic> json) {
     final response = AgentToolResponse.fromJson(json);
     callbacks.onAgentToolResponse?.call(response);

lib/src/models/callbacks.dart

@@ -53,6 +53,9 @@ class ConversationCallbacks {
   /// Called when MCP connection status changes
   final void Function(McpConnectionStatus status)? onMcpConnectionStatus;
 
+  /// Called when the agent is about to call a server-side tool (webhook, etc.)
+  final void Function(AgentToolRequest request)? onAgentToolRequest;
+
   /// Called when an agent tool response is received
   final void Function(AgentToolResponse response)? onAgentToolResponse;
 
@@ -94,6 +97,7 @@ class ConversationCallbacks {
     this.onUnhandledClientToolCall,
     this.onMcpToolCall,
     this.onMcpConnectionStatus,
+    this.onAgentToolRequest,
     this.onAgentToolResponse,
     this.onDebug,
     this.onEndCallRequested,

lib/src/models/events.dart

@@ -220,6 +220,39 @@ class McpIntegration {
   }
 }
 
+/// Agent tool request - sent when the agent is about to call a server-side tool
+class AgentToolRequest {
+  /// Tool name
+  final String toolName;
+
+  /// Tool call identifier
+  final String toolCallId;
+
+  /// Tool type
+  final String toolType;
+
+  /// Event identifier
+  final int eventId;
+
+  AgentToolRequest({
+    required this.toolName,
+    required this.toolCallId,
+    required this.toolType,
+    required this.eventId,
+  });
+
+  factory AgentToolRequest.fromJson(Map<String, dynamic> json) {
+    final agentToolRequest =
+        json['agent_tool_request'] as Map<String, dynamic>;
+    return AgentToolRequest(
+      toolName: agentToolRequest['tool_name'] as String,
+      toolCallId: agentToolRequest['tool_call_id'] as String,
+      toolType: agentToolRequest['tool_type'] as String,
+      eventId: agentToolRequest['event_id'] as int,
+    );
+  }
+}
+
 /// Agent tool response
 class AgentToolResponse {
   /// Tool name