Inconsistent spelling: "cancelled" vs "canceled" in session status strings

Problem

Currently the success alert after canceling a session says "Session has been cancelled successfully." (British spelling), while the status filter and badge labels use "Canceled" (American spelling). GitLab documentation style is American English, so user-facing strings should consistently use "canceled".

Affected files

• ee/app/assets/javascripts/ai/duo_agents_platform/pages/show/duo_agents_platform_show.vue ("Session has been cancelled successfully.")
• ee/app/assets/javascripts/ai/duo_agents_platform/components/common/agent_flow_user_approval.vue (same alert string)
• ee/app/assets/javascripts/ai/duo_agents_platform/panel/constants.js (STOPPED: Canceled)

Task

• Update user-facing session strings to consistently use the American spelling "canceled"