fix: Send raw content in webhook payloads instead of channel-rendered markdown (#13896)

Webhook payloads (`message_created`, `message_updated`) started sending
channel-rendered HTML in the `content` field instead of the original raw
message content after PR #12878. This broke downstream agent bots and
integrations that expected plain text or markdown. 
Closes https://linear.app/chatwoot/issue/PLA-109/webhook-payloads-send-channel-rendered-html-instead-of-raw-content

## How to reproduce

1. Connect an agent bot to a WebWidget inbox
2. Send a message with markdown formatting (e.g. `**bold**`) from the
widget
3. Observe the agent bot webhook payload — `content` field contains
`<p><strong>bold</strong></p>` instead of `**bold**`

## What changed

Split `MessageContentPresenter` into two public methods:
- `outgoing_content` — renders markdown for the target channel (used by
channel delivery services)
- `webhook_content` — returns raw content with CSAT survey URL when
applicable, no markdown rendering (used by `webhook_data`)

Updated `Message#webhook_data` to use `webhook_content` instead of
`outgoing_content`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>

app/presenters/message_content_presenter.rb

@@ -1,22 +1,28 @@
 class MessageContentPresenter < SimpleDelegator
   def outgoing_content
-    content_to_send = if should_append_survey_link?
-                        survey_link = survey_url(conversation.uuid)
-                        custom_message = inbox.csat_config&.dig('message')
-                        custom_message.present? ? "#{custom_message} #{survey_link}" : I18n.t('conversations.survey.response', link: survey_link)
-                      else
-                        content
-                      end
-
     Messages::MarkdownRendererService.new(
-      content_to_send,
+      content_with_survey_link,
       conversation.inbox.channel_type,
       conversation.inbox.channel
     ).render
   end
 
+  def webhook_content
+    content_with_survey_link
+  end
+
   private
 
+  def content_with_survey_link
+    if should_append_survey_link?
+      survey_link = survey_url(conversation.uuid)
+      custom_message = inbox.csat_config&.dig('message')
+      custom_message.present? ? "#{custom_message} #{survey_link}" : I18n.t('conversations.survey.response', link: survey_link)
+    else
+      content
+    end
+  end
+
   def should_append_survey_link?
     input_csat? && !inbox.web_widget?
   end