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/models/message.rb

@@ -176,7 +176,7 @@ class Message < ApplicationRecord
       additional_attributes: additional_attributes,
       content_attributes: content_attributes,
       content_type: content_type,
-      content: outgoing_content,
+      content: webhook_content,
       conversation: conversation.webhook_data,
       created_at: created_at,
       id: id,
@@ -195,6 +195,11 @@ class Message < ApplicationRecord
     MessageContentPresenter.new(self).outgoing_content
   end
 
+  # Raw content with survey URL (no markdown rendering) for webhook consumers
+  def webhook_content
+    MessageContentPresenter.new(self).webhook_content
+  end
+
   def email_notifiable_message?
     return false if private?
     return false if %w[outgoing template].exclude?(message_type)