[matrix-channel] Silent turns are labelled 「已完成」 ("Done") — a false completion signal in rooms

Problem

When an agent turn ends with no visible final text, the Matrix channel edits the
per-turn placeholder (「处理中...」) to 「已完成」("Done") — unconditionally.

There are two very different situations behind "no visible text", and the current code
conflates them:

1. Quiet completion — the turn did real work (tool calls) and simply had nothing to
   say. Marking it done is reasonable.
2. Silent/empty turn — the model produced no tool calls and no meaningful output at
   all (observed failure mode: a long reasoning-only response that ends without any
   tool call or final text; e.g. generation truncated at the output budget). The turn
   did nothing, yet the room is told 「已完成」.

In a production incident, five such silent turns (each ending with tens of KB of
reasoning-only output, zero tool calls) terminated with the 「已完成」 marker. Room
observers and the coordination layer treated them as successful completions — the
silence was upgraded from "invisible" to "believed done", and the failures were only
caught by human intervention hours later.

Where (code chain)

plugins/agentteams-matrix-channel/agentteams_matrix/channel.py →
_on_process_completed() (current main, ~L4450–4510):

• placeholder branch pops _MATRIX_PLACEHOLDER_THREAD_ROOT_KEY and edits the thread
  root with:
  • "已处理" when the final output is the NO_REPLY control line (good, L4472/L4490);
  • "已完成" whenever there is no visible text — three paths:
    • streaming text present but empty after control-text stripping (L4478);
    • pending final message present but empty after stripping (L4502);
    • no final text at all (L4506).
• The discriminator that distinguishes "quiet completion" from "did nothing" does not
  exist anywhere in the path.

(Historical note: the completion marker was introduced with the QwenPaw 2.0 adaptation,
fb3a40be.)

Impact

• False completion signal: room observers (team leader / humans / coordinating
  agents) see 「已完成」 and move on; a turn that produced nothing is recorded as done.
• Silent automation: no retry, no escalation, no alert — the failure is masked by
  the success marker, and rooms can stay silent for hours.
• Observability loss: the marker is the only per-turn outcome the room gets in
  the no-output case; it currently always says "done".

Proposal

Keep the marker, make it honest:

1. Track per-turn activity in the channel's per-turn state (send_meta):
   • whether any tool call / tool output message was routed during the turn
     (the channel already observes these events — they drive tool-message rendering);
   • whether any visible text (final or streamed) was produced.
2. In the placeholder-finalization branch, when the turn ends with
   zero tool calls && visible text ≤ ~10 chars (or empty) → post an explicit
   no-output marker (e.g. 「本回合无产出」) instead of 「已完成」.
   • NO_REPLY continues to map to 「已处理」 (unchanged);
   • turns with tool activity keep the current 「已完成」 behavior.
3. (Optional, follow-up) emit a machine-readable signal alongside the marker so the
   dispatching party can be re-woken automatically — turning multi-hour silent stalls
   into seconds-visible events.

Tests

• Unit: empty turn (no tool message + no text) → thread root edited to the no-output
  marker, not 「已完成」;
• Unit: tool-only turn with no text → 「已完成」 (unchanged);
• Unit: NO_REPLY → 「已处理」 (unchanged);
• Regression: normal final-text edit unchanged.

空转回合被标「已完成」——房间里的假完成信号

问题：回合结束时若「可见最终文本」为空，Matrix 频道会把占位消息（「处理中...」）无条件改成**「已完成」。但"没有可见文本"背后是两种完全不同的情况：①安静完成**（真干了活、调了工具、只是没话说）——标"已完成"合理；②空转回合（模型零工具调用、无有意义输出——已观测形态：整回合只有超长思维流、无任何工具调用与最终文本，如生成撞输出预算被截断）——什么也没做，房间却收到「已完成」。在生产事故中，5 个此类空转回合均以「已完成」收口：房间观察者与协调层把它当成功完成，静默从"不可见"升级为"被误认为已完成"，数小时后才被人工发现。

链条：plugins/agentteams-matrix-channel/agentteams_matrix/channel.py → _on_process_completed()（当前 main ~L4450–4510）：占位分支编辑线程根——NO_REPLY → 「已处理」（正确，L4472/L4490）；凡无可见文本 → 「已完成」（三条路径：L4478 / L4502 / L4506）。整条路径没有任何"做了事 vs 什么都没做"的判别。

影响：假完成信号 → 观察者（Leader/人类/协调 Agent）以为完成直接走开；无重试、无升级、无告警，房间可静默数小时；每回合唯一的产出标记永远说"完成"。

提案：把标记做诚实——① 在回合状态（send_meta）里记录：本回合是否路由过工具调用/工具输出消息（频道本就观察这些事件）、是否有过可见文本；② 占位收口分支：零工具 且 可见文本 ≤约 10 字（或为空） → 发明确的无产出标记（如「本回合无产出」）而非「已完成」；NO_REPLY 维持「已处理」；有工具活动的回合维持「已完成」。③（可选跟进）随标记发一个机器可读信号，供派发方自动重唤醒——把数小时静默压回秒级可见。

测试：空回合 → 无产出标记（非「已完成」）；无文本的工具回合 → 「已完成」不变；NO_REPLY → 「已处理」不变；正常文本收口回归不变。