feat(weixin): allow lightweight markdown formatting for improved message readability

[1000cm]

Summary

This PR updates the WeChat (Weixin) prompt to allow lightweight Markdown formatting for improved readability and structured responses.

Motivation

The current prompt explicitly restricts Markdown usage, which limits the model's ability to produce well-structured outputs (e.g., tables, lists, formatted summaries).

In practice, even partial Markdown significantly improves user experience, especially for structured data.

Changes

• Allow lightweight Markdown usage (headings, lists, bold text, tables)
• Remove strict "plain text only" limitation
• Ensure outputs remain readable even if Markdown is not rendered
• Add guidance to avoid overly complex formatting

Benefits

• Better structured and more readable responses
• Improved presentation of tabular and structured data
• Maintains backward compatibility with plain-text clients

Notes

This change is intentionally conservative:

• Markdown usage is optional and limited to simple constructs
• Outputs are designed to degrade gracefully if not rendered

[teknium1]

Merged via PR #8665 which synthesizes the best fixes from ~25 community PRs into a single consolidated change. Your contribution (lightweight markdown formatting) was reviewed and informed the final implementation. Thank you @1000cm for your work on this!