refactor(ai): remove hardcoded gpt-4o fallback model

Remove the hardcoded "gpt-4o" fallback in cmd_ai_start to ensure
model selection relies exclusively on provider configuration defaults.

src/command/cmd_funcs.c

@@ -10944,8 +10944,11 @@ cmd_ai_start(ProfWin* window, const char* const command, gchar** args)
     if (!model) {
         model = ai_get_provider_default_model(provider_name);
     }
+
     if (!model) {
-        model = "gpt-4o"; // Fallback
+        cons_show_error("No model specified and no default model set for provider '%s'.", provider_name);
+        cons_show("Use '/ai set default-model %s <model>' or '/ai switch %s <model>'.", provider_name, provider_name);
+        return TRUE;
     }
 
     // Check for API key