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

spec/services/messages/markdown_renderer_service_spec.rb

@@ -74,6 +74,24 @@ RSpec.describe Messages::MarkdownRendererService, type: :service do
         expect(result.scan("\n").count).to eq(4)
         expect(result).to include("Para 1\n\n\n\nPara 2")
       end
+
+      it 'renders code blocks as plain text' do
+        content = "```\ncode here\n```"
+        result = described_class.new(content, channel_type).render
+        expect(result.strip).to eq('code here')
+      end
+
+      it 'renders indented code blocks as plain text preserving exact content' do
+        content = '    indented code line'
+        result = described_class.new(content, channel_type).render
+        expect(result).to eq('indented code line')
+      end
+
+      it 'handles code blocks with emojis and special characters without stack overflow' do
+        content = "    first line\n    🌐 second line\n"
+        result = described_class.new(content, channel_type).render
+        expect(result).to eq("first line\n🌐 second line")
+      end
     end
 
     context 'when channel is Channel::Instagram' do
@@ -130,6 +148,24 @@ RSpec.describe Messages::MarkdownRendererService, type: :service do
         expect(result.scan("\n").count).to eq(4)
         expect(result).to include("Para 1\n\n\n\nPara 2")
       end
+
+      it 'renders code blocks as plain text' do
+        content = "```\ncode here\n```"
+        result = described_class.new(content, channel_type).render
+        expect(result.strip).to eq('code here')
+      end
+
+      it 'renders indented code blocks as plain text preserving exact content' do
+        content = '    indented code line'
+        result = described_class.new(content, channel_type).render
+        expect(result).to eq('indented code line')
+      end
+
+      it 'handles code blocks with emojis and special characters without stack overflow' do
+        content = "    first line\n    🌐 second line\n"
+        result = described_class.new(content, channel_type).render
+        expect(result).to eq("first line\n🌐 second line")
+      end
     end
 
     context 'when channel is Channel::Line' do
@@ -358,6 +394,18 @@ RSpec.describe Messages::MarkdownRendererService, type: :service do
         expect(result).to include('1. first step')
         expect(result).to include('2. second step')
       end
+
+      it 'renders code blocks as plain text' do
+        content = "```\ncode here\n```"
+        result = described_class.new(content, channel_type).render
+        expect(result.strip).to eq('code here')
+      end
+
+      it 'handles code blocks with emojis and special characters without stack overflow' do
+        content = "    first line\n    🌐 second line\n"
+        result = described_class.new(content, channel_type).render
+        expect(result).to eq("first line\n🌐 second line")
+      end
     end
 
     context 'when channel is Channel::TwilioSms' do