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 message but previously hit the default
case, logging "Unknown event type: agent_tool_request". This is a valid
event type that should be handled. Since it is informational only (the
agent_tool_response already handles the result), we just pass it to
the debug callback.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: seahyc

lib/src/messaging/message_handler.dart

@@ -96,6 +96,11 @@ class MessageHandler {
           _handleMcpConnectionStatus(json);
           break;
 
+        case 'agent_tool_request':
+          // Agent is about to call a tool (webhook, etc.) - informational only
+          callbacks.onDebug?.call(json);
+          break;
+
         case 'agent_tool_response':
           callbacks.onDebug?.call(json);
           _handleAgentToolResponse(json);