fix(chat): request cancellation over HTTP without optimistic completion

[dangzitou]

Summary

Fixes #803 for ordinary running Dashboard turns.

• Add an authenticated POST /agents/{agent_id}/threads/{thread_id}/cancel endpoint using the existing thread ownership check and synchronous cancellation entry.
• Send Stop requests over HTTP with a separate 10-second timeout and an in-flight click guard, without requiring an existing WebSocket. Do not automatically retry.
• Report requested / inactive / unconfirmed outcomes in English and Chinese. Keep stream reception, messages, usage, and reconnect state intact until the existing terminal/state-sync path observes completion; re-subscribe when no live socket remains.
• Add cancellation regression coverage and update the changelog and API reference.

requested: true acknowledges that the cancellation entry was called, not that the task has already stopped. This does not address queued tasks, HITL, exact earlier-turn cancellation, rollback of external side effects, or Harness-internal cancellation (#805). Legacy WebSocket cancel and disconnect behavior are unchanged.

Target branch

• Base is develop (feature / fix — default)
• Base is main (release/* or hotfix/* only)

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Refactor / chore
• Release / hotfix

Test plan

• Local rebase verification at ec70e5f4: the Unreleased changelog conflict was resolved, the functional patch kept its stable patch-id, and git diff --check passed. Earlier local WebSocket integration 21 passed, stop-button tests 10 passed, and dashboard build passed. The latest Linux / Windows result is shown in GitHub Checks.
• Previous rebase onto develop at 963cb140 (head d9032fe9): functional diff unchanged; WebSocket integration 21 passed, cancellation Vitest 10 passed, and npm run build passed.
• On develop at 728ca521, make all PYTEST_JOBS=2 passed Ruff, formatting and mypy; the full backend run had 3713 passed, 18 skipped, 1 failed. The failure was the existing test_process_exit_after_commit_keeps_last_fragment subprocess exceeding its 20-second timeout while multiple suites were running.
• After rebasing onto 6f7815bb (head ded4854e), the timeout test passed alone without code or timeout changes. make lint typecheck passed; uv run pytest tests/integration/test_chat_ws.py tests/unit/i18n -n 2 -q passed (91 passed).
• Previous-head Vitest coverage passed: 16 tests across cancellation, ChatInput prefill and connector-card suites. Cancellation covers missing/closed WebSocket, authentication and encoded IDs, duplicate clicks, timeout/error feedback, retained streaming/usage, terminal handling, and session identity after navigation.
• npm run build passed, including TypeScript checking and the production build.
• OpenAPI response schema and Scalar rendering were inspected during implementation.

The full local run above was not green; its sole timeout passed in isolation. The unchanged chatStore.ts also has the same no-extra-boolean-cast ESLint error as clean develop; no unrelated lint fix is included. Local checks used macOS / Python 3.14. Final-head Linux/Windows CI and maintainer testing are tracked separately; real-LLM/manual refresh-and-stop testing is not claimed.

Reproduction evidence (before / after)

Turn running, dashboard without a live WebSocket: on develop POST /agents/{id}/threads/{tid}/cancel → 404 and the turn keeps streaming; on this branch it returns 200 {"requested": true} and the turn terminates (idle re-post honestly reports requested: false).

Before (develop @ 4667bd7b):

After (this branch):

Checklist

• Updated CHANGELOG.md (if user-facing)
• README / docs updated (if needed)

[jubaoliang]

问题是真的：Stop 只走 WebSocket，并且本地会乐观把回合标成 done。补 HTTP cancel、等服务端回执再结束，方向对。

当前与 develop 冲突（useChat 等）。请 rebase 到最新 develop，CI 绿后再合。夹带的无关测试修如果不是取消路径必需，请一并拿掉。

[dangzitou]

@jubaoliang 已按意见处理，麻烦复核：

• 已 rebase 到最新 develop（ae7dfc8b），新提交 168f1863，当前 GitHub 显示 MERGEABLE、无冲突。冲突解决未改动修复逻辑：CHANGELOG.md 保留双方条目；useChat.ts 里 develop 新增的 setSessionTeamRoom effect 与本次改动并存，没有回退 develop 的代码。
• 与取消路径无关的测试修改已全部移除（publishedExperts.test.ts、AuthGuard.test.tsx、Channels/components/constants.test.ts、Settings/SearchConfig/index.test.tsx、test/setup.ts）。diff 现在只含取消链路本身：HTTP 接口、chatStore / useChat、octopThreads、新增 useChat.cancel.test.ts、中英文文案、docs/api.md 与 CHANGELOG，共 11 个文件。
• 本地验证：完整非 live 后端套件 3628 passed / 17 skipped，Ruff 与 mypy --strict 通过；tests/integration/test_chat_ws.py 21 项全过（含新增的 HTTP cancel 两项）；前端完整套件结果与干净 develop 基线完全一致（develop 自身就有同样 4 个文件 / 1 个测试失败，均为既有问题），新增 useChat.cancel.test.ts 10 项全过；tsc -b 与 Prettier 通过。

fork PR 的工作流需要审批才会运行，烦请批准后跑一遍 CI；若复核无问题，麻烦合并到 develop。谢谢！

[dangzitou]

@jubaoliang 已 rebase 到最新拉取的 develop（6f7815bb），当前 head 为 ded4854e。保留现有流结束/状态同步语义，停止按钮只通过 HTTP 发起取消，不提前把运行状态改成完成；功能 diff 未变，没有混入无关前端修复。

验证如实说明：首轮 make all 静态检查通过，全量 3713 passed / 18 skipped，唯一失败是已有历史持久化子进程的 20 秒超时。最终 head 上单独复跑该用例通过，未改代码或超时；后端相关回归 91 passed、前端相关回归 16 passed，lint / mypy / TypeScript / 生产构建通过。chatStore.ts 的既有 ESLint 报错在 develop 原文件上同样可复现，未混入修复。最新 Linux / Windows CI 等待本次推送后的结果。

PR 描述已更新验证范围。请按你的节奏测试，后续我会继续跟进 develop 并及时 rebase。

[dangzitou]

@jubaoliang 同步到新 develop（963cb140），head d9032fe9。CHANGELOG 冲突已合并，取消链路的功能 diff 逐行未变。本地 WebSocket 集成回归 21 passed、停止按钮回归 10 passed，TypeScript 与生产构建通过。新 head 的 Linux / Windows CI 正在运行，我会继续跟进。

[dangzitou]

@jubaoliang 这个是自己用得最多的：点停止前端会把回合乐观标成完成，帧也照收，其实服务端还没取消完。

按你说的，rebase 和不相关的测试修都处理了，现在只剩取消链路本身——HTTP 发起取消，等服务端回执再结束。刚跟着最新 develop 又推了一版。

正文最底下有修复前后的验证截图，麻烦再审一遍。