fix(weixin): send single message per response instead of splitting

[Liuwei1125]

Summary

优化微信消息回复策略：将原来按换行/markdown 单元拆分多条消息的方式，改为"一次回复 = 一条消息"。

Problem

之前的 _split_text_for_weixin_delivery 会把一条回复按换行、markdown 单元拆成多条消息发送，导致用户体验碎片化。

Solution

• 短内容（≤ max_length）：直接发送单条消息
• 超长内容：截断（truncate）而非拆分，保留可读性

- Prefer one message per top-level line/markdown unit when the author used
- explicit line breaks. Oversized units fall back to block-aware packing so
- long code fences still split safely.
+ A single response = a single Weixin message.  Only split when the content
+ genuinely exceeds the API limit.  Oversized content is truncated (not
+ broken into multiple messages) to preserve readability.

[teknium1]

Merged via PR #8665 which synthesizes the best fixes from ~25 community PRs into a single consolidated change. Your contribution (single message per response) was reviewed and informed the final implementation. Thank you @Liuwei1125 for your work on this!