feat: preview Agent HTML in conversations and unblock long replies

Agent replies that produced HTML could only be read as source, so users had
to copy the markup into an external file to see the result. Completed Agent
and Subagent output now renders complete HTML documents, single-root HTML
replies, and `html` / `htm` code blocks as an isolated static preview in
place. Each preview keeps a persistent notice that scripts and network are
disabled and offers the same icon actions as Mermaid diagrams: a source
toggle and a full-screen preview. The new "render HTML in conversations"
switch under Platform Features / General is on by default, applies to the
open conversation immediately, and falls back to HTML source when disabled.
Previews cannot run scripts, reach the network, submit forms, open windows,
or load remote media; author stylesheets and inline styles are preserved so
the intended layout survives, and remote resource loading is refused by the
preview CSP instead.

Direct-model replies no longer fail once they grow large. The byte caps on
streaming responses, single stream blocks, non-streaming replies, model tool
arguments, tool request context, aggregate tool results, and accumulated
answer text are removed, so users stop seeing "模型接口流式响应超过安全限制"
on long answers. Response length is now governed by the request timeout,
cancellation, and context compression. Image generation responses keep their
decodable-image validation.

A message sent after a failed or interrupted run stayed in the pending queue
instead of running. The conversation is now released whenever a run reaches
any terminal state, not only when the renderer reports readiness again, and a
dispatched queue item whose run never starts releases its reservation and
returns to pending. Later messages dispatch without extra user action.

Release note: Agent 回复中的 HTML 现在可直接在会话内预览，支持查看源码和全屏预览，默认开启并可在“平台功能 / 通用设置”关闭；直连模型的长回复不再因大小限制中断；上一次运行失败或被中断后，新发送的消息会立即执行而不再排队等待。

docs/features/direct-model-agent/technical-design.md

@@ -327,11 +327,15 @@ type SubagentActor =
 
 不要用宽泛 catch 抹掉取消、超时、Provider HTTP 状态或进程退出上下文。
 
-模型请求重试由 `ModelAgentRuntime` 的统一流式请求边界处理，覆盖普通问答、工具轮次和复用
+模型请求重试由 `ModelAgentRuntime` 的统一流式请求处理，覆盖普通问答、工具轮次和复用
 同一 Runtime 的上下文摘要。重试分类检查错误及有界 `cause` 链中的 Node/Undici 网络错误
 码；每次尝试创建独立超时信号。退避等待监听父请求取消。只有完整模型轮次返回后才执行
 工具，因此未产生可见文本或推理内容的重试不会重复执行工具。
 
+模型回复本身不设固定字节上限：流式响应、非流式回复、模型工具参数、请求上下文和工具
+结果总量都由请求超时、取消和上下文压缩控制，长回复和大工具结果不会被中断。图像生成
+响应仍按可解码图片大小校验。
+
 ## 11. 性能
 
 - Shell 探测按执行服务实例缓存，不在每轮模型调用重复探测。