feat: Add activity messages for linear actions (#11654)

app/services/linear/activity_message_service.rb

@@ -0,0 +1,41 @@
+class Linear::ActivityMessageService
+  attr_reader :conversation, :action_type, :issue_data, :user
+
+  def initialize(conversation:, action_type:, user:, issue_data: {})
+    @conversation = conversation
+    @action_type = action_type
+    @issue_data = issue_data
+    @user = user
+  end
+
+  def perform
+    return unless conversation && issue_data[:id] && user
+
+    content = generate_activity_content
+    return unless content
+
+    ::Conversations::ActivityMessageJob.perform_later(conversation, activity_message_params(content))
+  end
+
+  private
+
+  def generate_activity_content
+    case action_type.to_sym
+    when :issue_created
+      I18n.t('conversations.activity.linear.issue_created', user_name: user.name, issue_id: issue_data[:id])
+    when :issue_linked
+      I18n.t('conversations.activity.linear.issue_linked', user_name: user.name, issue_id: issue_data[:id])
+    when :issue_unlinked
+      I18n.t('conversations.activity.linear.issue_unlinked', user_name: user.name, issue_id: issue_data[:id])
+    end
+  end
+
+  def activity_message_params(content)
+    {
+      account_id: conversation.account_id,
+      inbox_id: conversation.inbox_id,
+      message_type: :activity,
+      content: content
+    }
+  end
+end