## Description
Fixes an issue where multiple newlines with whitespace between them
(e.g., `\n \n \n`) were being collapsed to single newlines in text-based
messaging channels (Telegram, WhatsApp, Instagram, Facebook, Line, SMS).
The frontend was sending messages with spaces/tabs between newlines, and
the markdown renderer was treating these as paragraph content,
collapsing them during rendering.
### Changes:
1. Added whitespace normalization in `render_telegram_html`,
`render_whatsapp`, `render_instagram`, `render_line`, and
`render_plain_text` methods
2. Strips whitespace from whitespace-only lines before markdown
processing
3. Added comprehensive regression tests for all affected channels
## Type of change
- [x] Bug fix (non-breaking change which fixes an issue)
## How Has This Been Tested?
1. **Unit Tests**: Added 7 new specs testing multiple newlines with
whitespace between them for all text-based channels
2. **Manual Testing**: Verified with actual frontend payload containing
`\n \n \n` patterns
3. **Regression Testing**: All existing 63 specs pass
### Test Results:
- ✅ All 63 markdown renderer specs pass (56 original + 7 new)
- ✅ All 12 Telegram channel specs pass
- ✅ All 27 WhatsApp + Instagram specs pass
- ✅ Verified with real-world payload: 18 newlines preserved (previously
collapsed to 1)
### Test Command:
```bash
RAILS_ENV=test bundle exec rspec spec/services/messages/markdown_renderer_service_spec.rb
RAILS_ENV=test bundle exec rspec spec/models/channel/telegram_spec.rb
```
## Checklist:
- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective
- [x] New and existing unit tests pass locally with my changes
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