## Description
This PR fixes an issue where multiple consecutive newlines (blank lines
for visual spacing) were being collapsed in text-based messaging
channels like WhatsApp, Instagram, and SMS.
When users send messages via API with intentional spacing using multiple
newlines (e.g., `\n\n\n\n`), the markdown renderer was following
standard Markdown spec and collapsing them into single blank lines.
While this is correct for document formatting, messaging platforms like
WhatsApp and Instagram support and preserve multiple blank lines for
visual spacing.
The fix adds preprocessing to preserve multiple consecutive newlines
(3+) by converting them to placeholder tokens before CommonMarker
processing, then restoring the exact number of newlines in the final
output.
## Changes
- Added `preserve_multiple_newlines` and `restore_multiple_newlines`
helper methods to `MarkdownRendererService`
- Updated `render_whatsapp` to preserve multiple consecutive newlines
- Updated `render_instagram` to preserve multiple consecutive newlines
- Updated `render_plain_text` (affects SMS, Twilio SMS, Twitter) to
preserve multiple consecutive newlines
- Updated `render_line` to preserve multiple consecutive newlines
- HTML-based renderers (Email, Telegram, WebWidget) remain unchanged as
they handle spacing via HTML tags
## Type of change
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality not to work as expected)
- [ ] This change requires a documentation update
## How Has This Been Tested?
Added comprehensive test coverage:
- 3 new tests for multi-newline preservation across WhatsApp, Instagram,
and SMS channels
- All 56 tests passing (up from 53)
Testing scenarios:
- Single newlines preserved: `"Line 1\nLine 2"` remains `"Line 1\nLine
2"`
- Multiple newlines preserved: `"Para 1\n\n\n\nPara 2"` remains `"Para
1\n\n\n\nPara 2"`
- Standard paragraph breaks (2 newlines) work as before
- Markdown formatting (bold, italic, links) continues to work correctly
- Backward compatibility maintained for all channels
## Checklist:
- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [x] Any dependent changes have been merged and published in downstream
modules
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