- Fix double bold markers (****) when heading text already contains **
- Strip markdown formatting (**bold**, *italic*, ~~strike~~) from table cells
since Feishu table elements do not support markdown rendering
Fixes rendering issues where:
1. Headings like '**text**' were rendered as '****text****'
2. Table cells with '**bold**' showed raw markdown instead of plain text
- Add VolcEngine ProviderSpec entry in registry.py
- Add volcengine to ProvidersConfig class in schema.py
- Update model providers table in README.md
- Add description about VolcEngine coding plan endpoint