fix: Preserve double newlines in text-based messaging channels (#13055)

## Summary

Fixes the issue where double newlines (paragraph breaks) were collapsing
to single newlines in text-based messaging channels (Telegram, WhatsApp,
Instagram, Facebook, LINE, SMS).

### Root Cause

The `preserve_multiple_newlines` method only preserved 3+ consecutive
newlines using the regex `/\n{3,}/`. When users pressed Enter twice
(creating a paragraph break with 2 newlines), CommonMarker would parse
this as separate paragraphs, which then collapsed to a single newline in
the output.

This caused:
- ❌ Normal Enter: Double newlines collapsed to single newline
- ✅ Shift+Enter: Worked (created hard breaks)

### Fix

Changed the regex from `/\n{3,}/` to `/\n{2,}/` to preserve 2+
consecutive newlines. This prevents CommonMarker from collapsing
paragraph breaks.

Now:
- ✅ Single newline (`\n`) → Single newline (handled by softbreak)
- ✅ Double newline (`\n\n`) → Double newline (preserved with
placeholders)
- ✅ Triple+ newlines → Preserved as before

### Test Coverage

Added comprehensive tests for:
- Single newlines preservation
- Double newlines (paragraph breaks) preservation  
- Multiple consecutive newlines
- Newlines with varying amounts of whitespace between them (1 space, 3
spaces, 5 spaces, tabs)

All 66 tests passing.

### Impact

This fix affects all text-based messaging channels that use the markdown
renderer:
- Telegram
- WhatsApp
- Instagram  
- Facebook
- LINE
- SMS
- Twilio SMS (when configured for WhatsApp)

Fixes
https://linear.app/chatwoot/issue/CW-6135/double-newline-is-breaking

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

app/services/messages/markdown_renderer_service.rb

@@ -96,10 +96,10 @@ class Messages::MarkdownRendererService
     restore_multiple_newlines(result)
   end
 
-  # Preserve multiple consecutive newlines (3+) by replacing them with placeholders
-  # Standard markdown treats 2 newlines as paragraph break, we preserve 3+
+  # Preserve multiple consecutive newlines (2+) by replacing them with placeholders
+  # Standard markdown treats 2 newlines as paragraph break which collapses to 1 newline, we preserve 2+
   def preserve_multiple_newlines(content)
-    content.gsub(/\n{3,}/) do |match|
+    content.gsub(/\n{2,}/) do |match|
       "{{PRESERVE_#{match.length}_NEWLINES}}"
     end
   end