fix(weixin): wrap long copy-unfriendly lines

[LeonSGP43]

Summary

• Wraps long non-code, non-table Weixin text lines at a copy-friendly display width.
• Leaves fenced code blocks and Markdown tables untouched so commands/code and table structure are not damaged.
• Adds focused Weixin formatting regressions for long line wrapping and code-block preservation.

Why

Fixes #19161. WeChat clients can make very long single-line Hermes replies difficult to copy, especially issue-template or log-style prose. Breaking long display text at natural whitespace keeps the full content while making it easier to select and copy.

Scope

This only changes outbound Weixin formatting before the existing message chunking path. It does not alter Weixin delivery, message limits, media handling, or other gateway platforms.

Verification

• scripts/run_tests.sh tests/gateway/test_weixin.py -> 51 passed, 4 warnings
• git diff --check