fix: Add code_block method to WhatsApp and Instagram markdown renderers (#13166)

Problem: SystemStackError: stack level too deep occurred when rendering
messages with indented content (4+ spaces) for WhatsApp, Instagram, and
Facebook channels.

Root Cause: CommonMarker::Renderer#code_block contains a self-recursive
placeholder that must be overridden:
```
  def code_block(node)
    code_block(node)  # calls itself infinitely
  end
```

WhatsAppRenderer and InstagramRenderer were missing this override,
causing infinite recursion when markdown with 4-space indentation
(interpreted as code blocks) was rendered.

Fix: Added code_block method to both renderers that outputs the node
content as plain text:
```
  def code_block(node)
    out(node.string_content)
  end
 ```

Fix https://linear.app/chatwoot/issue/CW-6217/systemstackerror-stack-level-too-deep-systemstackerror

app/services/messages/markdown_renderers/instagram_renderer.rb

@@ -42,6 +42,10 @@ class Messages::MarkdownRenderers::InstagramRenderer < Messages::MarkdownRendere
     cr
   end
 
+  def code_block(node)
+    out(node.string_content)
+  end
+
   def softbreak(_node)
     out("\n")
   end

app/services/messages/markdown_renderers/whats_app_renderer.rb

@@ -30,6 +30,10 @@ class Messages::MarkdownRenderers::WhatsAppRenderer < Messages::MarkdownRenderer
     cr
   end
 
+  def code_block(node)
+    out(node.string_content)
+  end
+
   def softbreak(_node)
     out("\n")
   end