fix: serialize multimodal Captain session content (#15096)

Captain now saves agent session records when a user message includes an
image. The saved record keeps the image URL and excludes downloaded
image bytes, so image replies no longer report a JSON serialization
error after delivery.

Fixes:
https://chatwoot-p3.sentry.io/issues/7618423184/?alert_rule_id=13673680&alert_type=issue&notification_uuid=d22a7ab9-95d6-4bba-85e0-733a28466775&project=6382945

## Root cause

RubyLLM downloads image attachments and caches the binary bytes inside
`RubyLLM::Content`. `SessionCaptureService` passed the live object to
the `run_context` JSON column. Rails then tried to encode the cached
JPEG bytes as UTF-8 and raised `JSON::GeneratorError`.

The error did not block replies, handoffs, or credit updates because
session capture rescues its own failures. The failed write meant that
Chatwoot lost the agent session record for the response.

## How to reproduce

1. Send an image to a Captain V2 assistant.
2. Let RubyLLM load the image during the model request.
3. Save the resulting conversation history in an agent session.
4. Observe the JSON encoding error when Rails reaches the cached image
bytes.

## What changed

`SessionCaptureService` now converts `RubyLLM::Content` to its JSON safe
hash before saving the current turn. The hash contains the message text
and attachment URL without the cached bytes. Other message content is
unchanged.

The focused service spec covers a cached JPEG byte payload and passes
with 12 examples. RuboCop reports no offenses in the changed service and
spec.

enterprise/app/services/captain/assistant/session_capture_service.rb

@@ -59,6 +59,11 @@ class Captain::Assistant::SessionCaptureService
   def current_turn_history
     history = Array(context[:conversation_history])
     last_user_index = history.rindex { |message| message[:role].to_s == 'user' }
-    last_user_index ? history[last_user_index..] : history
+    current_turn = last_user_index ? history[last_user_index..] : history
+
+    current_turn.map do |message|
+      content = message[:content]
+      content.is_a?(RubyLLM::Content) ? message.merge(content: content.to_h) : message
+    end
   end
 end

spec/enterprise/services/captain/assistant/session_capture_service_spec.rb

@@ -111,6 +111,25 @@ RSpec.describe Captain::Assistant::SessionCaptureService do
       expect(history.first).to include('role' => 'user', 'content' => 'CUST001')
     end
 
+    it 'stores multimodal content without cached attachment bytes' do
+      content = RubyLLM::Content.new('See image', ['https://example.com/image.jpg'])
+      content.attachments.first.instance_variable_set(:@content, "\xFF\xD8\xFF\xE0JFIF".b)
+      run_context[:conversation_history] = [
+        { role: :user, content: content },
+        { role: :assistant, content: 'I can see the image', agent_name: 'Assistant' }
+      ]
+
+      history = service.capture!.run_context
+
+      expect(history.first).to include(
+        'role' => 'user',
+        'content' => {
+          'text' => 'See image',
+          'attachments' => [{ 'type' => 'image', 'source' => 'https://example.com/image.jpg' }]
+        }
+      )
+    end
+
     it 'stores the full history when it contains no user message' do
       run_context[:conversation_history] = conversation_history.reject { |message| message[:role] == :user }