fix(wechat): suppress buffered output after /stop

[imMamdouhaboammar]

Problem

Upstream issue #396 reports that WeChat /stop aborts the agent but the handler keeps draining buffered turn output. Current main already labels the final state [已停止], but it still sends buffered text after the stop flag and can send files referenced by the cancelled result.

Root cause

on_message() used a per-user abort flag, but _handle() did not observe it until after its display-queue loop. That allowed buffered turn text and cancelled-result files to escape after /stop. Review also exposed two edge cases: an idle /stop could leave a stale flag for the next task, and setting the flag after agent.abort() left a small race where the worker could consume done before the abort became visible.

Fix

• only arm the per-user abort flag when the agent is actually running; clear stale flags for idle /stop
• arm _task_aborted[uid] before calling agent.abort()
• refuse incremental _send() calls once the abort flag is set
• stop draining the display queue as soon as an abort is observed
• after sending one [已停止] acknowledgement, return immediately so cancelled result files are not delivered

The task queue, WeChat protocol, authentication, and agent abort implementation are unchanged.

Regression coverage

tests/test_wechat_stop.py uses a fake agent and fake bot, with no WeChat network/login or LLM calls. Import stubs exist only during wechatapp.py import and sys.path/HOME are restored immediately. It covers:

1. buffered turn output queued after /stop is not sent
2. a real file referenced by the cancelled done result is not sent
3. /stop while idle does not cancel the next task
4. the abort flag is already armed at the instant agent.abort() runs

TDD verification

Original issue #396 validation:

• RED 31165947230: compile/import passed; both post-stop regressions failed on current main
• GREEN 31166076487: minimal fix passed compile, both regressions, and git diff --check

Review finding, stale idle stop:

• RED 31166842462: the two original tests passed; only the idle-stop regression failed
• GREEN 31166942104: all three tests, compile, and git diff --check passed

Abort ordering race:

• RED 31167274233: three existing tests passed; ordering test observed the flag as False inside agent.abort()
• GREEN 31167408972: all four tests, compile, and git diff --check passed after arming the flag first

The final PR commit is rebuilt directly from main using the exact production/test blobs from the last GREEN validation, so the contribution remains one commit and two changed files.

Scope

• frontends/wechatapp.py: focused /stop handling changes only
• one focused unittest file
• no new runtime dependencies

Fixes #396