fix(ai): resolve UAF and require provider argument instead of defaults

This enforces explicit provider specification.
Automatic fallback to preferences or "openai" is removed.
Users must now pass the provider name explicitly.

default provider functionality was improperly removed and implemented (some parts still remain intact), leading to UAF.

src/command/cmd_funcs.c

@@ -10932,21 +10932,14 @@ cmd_ai_start(ProfWin* window, const char* const command, gchar** args)
     if (g_strv_length(args) >= 2) {
         provider_name = args[1];
     }
-    if (g_strv_length(args) >= 3) {
-        model = args[2];
+
+    if (!provider_name) {
+        cons_show_error("Please specify provider name.");
+        return TRUE;
     }
 
-    // Resolve defaults
-    auto_gchar gchar* owned_provider_name = NULL;
-    if (!provider_name) {
-        // Use default provider from preferences
-        auto_gchar gchar* default_provider = prefs_get_string(PREF_AI_PROVIDER);
-        if (default_provider && strlen(default_provider) > 0) {
-            provider_name = default_provider;
-        }
-    }
-    if (!provider_name) {
-        provider_name = "openai"; // Fallback
+    if (g_strv_length(args) >= 3) {
+        model = args[2];
     }
 
     AIProvider* provider = ai_get_provider(provider_name);