test: mock LLM records system prompt for E2E assertions

Co-authored-by: CodeFree <codefree@chinatelcom.cn>

e2e/global-setup.ts

@@ -53,6 +53,12 @@ function startMockLlm(): Promise<void> {
           let parsed: any = {};
           try { parsed = JSON.parse(body); } catch { parsed = {}; }
           const model = parsed.model || "mock-model";
+          const systemMsg = Array.isArray(parsed.messages)
+            ? parsed.messages.find((m: any) => m.role === "system")
+            : undefined;
+          if (systemMsg?.content) {
+            (globalThis as any).__lastMockSystemPrompt = systemMsg.content;
+          }
           const chars = Array.from(MOCK_REPLY);
           res.writeHead(200, {
             "Content-Type": "text/event-stream",