refactor: 精简发给模型的上下文、隐藏接口原始报错，合并重复的删除逻辑

- 发给模型的对话只留最近 20 条，除最后一条 AI 回复外旧回复的代码块换成占位文字，
  对话越长越省 token，也不会超出上下文长度
- 学生看到的 AI 报错只显示超时 / 繁忙 / 出错（状态码），完整错误写日志；
  客户端自己断开导致的中止不再记错误
- 管理员查询对话必须带 task_id 或 user_id，不再一次查出全站所有对话
- “已评分或获奖不能删”和“删除一轮对话”合并到 services/cascade.ts，补上删除一轮对话的测试

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/tests/api.test.ts

@@ -226,6 +226,25 @@ describe("提交与删除", () => {
     expect(all(`SELECT * FROM prompt_message`)).toEqual([])
   })
 
+  test("删除一轮对话：连带提问和关联提交；被评过分的不能删", async () => {
+    makeUser("stu")
+    makeUser("other")
+    const task = makeTask("T")
+    const stu = await client("stu")
+    const first = await stu("POST", "/api/submission/", { task_id: task, html: "<p>1", prompt: "第一版" })
+    const second = await stu("POST", "/api/submission/", { task_id: task, html: "<p>2", prompt: "第二版" })
+    const asstOf = (sub: string) =>
+      get(`SELECT id FROM prompt_message WHERE submission_id = $id`, { id: sub.replaceAll("-", "") })!.id
+
+    await (await client("other"))("PUT", `/api/submission/${second.json.id}/score`, { score: 4 })
+    expect((await stu("DELETE", `/api/prompt/messages/${asstOf(second.json.id)}/pair`)).status).toBe(400)
+
+    const res = await stu("DELETE", `/api/prompt/messages/${asstOf(first.json.id)}/pair`)
+    expect(res.json).toEqual({ deleted: true, submission_deleted: true })
+    expect(all(`SELECT content FROM prompt_message WHERE role = 'user'`)).toEqual([{ content: "第二版" }])
+    expect(all(`SELECT COUNT(*) AS n FROM submission_submission`)).toEqual([{ n: 1 }])
+  })
+
   test("被评分的提交只有超管能删", async () => {
     const author = makeUser("author")
     makeUser("other")

server/tests/unit.test.ts

@@ -1,6 +1,13 @@
 import { describe, expect, test } from "bun:test"
 import { pyRound, pySum } from "../src/services/pyround"
-import { chatModel, extractCode, parseGuidanceResponse, streamChat } from "../src/services/llm"
+import {
+  chatModel,
+  extractCode,
+  parseGuidanceResponse,
+  streamChat,
+  trimHistory,
+  userFacingError,
+} from "../src/services/llm"
 import { config } from "../src/config"
 import { checkPassword, makePassword } from "../src/auth"
 import { formatMicros, iso, uuidHex, uuidOut } from "../src/db"
@@ -114,3 +121,31 @@ describe("streamChat", () => {
     expect(chatModel("")).toBe(config.llmModel)
   })
 })
+
+test("trimHistory 只留最近 20 条，旧回复的代码换成占位", () => {
+  const history = Array.from({ length: 30 }, (_, i) => ({
+    role: (i % 2 ? "assistant" : "user") as "user" | "assistant",
+    content: i % 2 ? `第${i}版\n\`\`\`html\n<p>${i}</p>\n\`\`\`` : `改一下 ${i}`,
+  }))
+  const out = trimHistory(history)
+  expect(out).toHaveLength(20)
+  expect(out[0]!.content).toBe("改一下 10")
+  expect(out.at(-1)!.content).toContain("<p>29</p>")
+  expect(out.at(-3)!.content).toBe("第27版\n```\n（旧版本代码已省略）\n```")
+  expect(out.filter((m) => m.content.includes("<p>"))).toHaveLength(1)
+})
+
+test("userFacingError 不透出接口原始返回", () => {
+  const quiet = console.error
+  console.error = () => {}
+  try {
+    expect(userFacingError(new Error('Error code: 401 - {"error":"invalid api key sk-xxx"}'))).toBe(
+      "AI 服务出错（401），请稍后重试",
+    )
+    expect(userFacingError(new Error("Error code: 429 - busy"))).toBe("AI 服务繁忙，请稍后重试")
+    expect(userFacingError(new Error("模型响应超时"))).toBe("AI 响应超时，请重试")
+    expect(userFacingError(new Error("fetch failed: ECONNREFUSED 10.0.0.1"))).toBe("AI 服务出错，请稍后重试")
+  } finally {
+    console.error = quiet
+  }
+})