fix(ns): root-qualify PHP builtins after PSR-4 move

Namespaced classes resolved DateTimeImmutable etc. under AKSRT\*
(e.g. Credits\DateTimeImmutable). Prefix with \ globally.

src/AKSRT/Ai/AI.php

@@ -282,7 +282,7 @@ class AI {
                 'success' => true,
                 'content' => $text,
             ];
-        } catch (Throwable $t) {
+        } catch (\Throwable $t) {
             return [
                 'success' => false,
                 'message' => $t->getMessage() ?: __('WordPress AI Client 调用失败', 'aksrt'),