Fix inconsistent spelling of canceled in session status

What does this MR do and why?

Fixes inconsistent spelling in user-facing session status strings. The success alert shown after canceling a Duo Agent Platform session used the British spelling "cancelled", while the rest of the UI uses the American spelling "canceled".

This change updates the alert message to use the American spelling "canceled" for consistency with the surrounding product copy (for example, the STOPPED status label already uses "Canceled"). It updates the two components that show the alert, the matching Jest spec, and the regenerated locale/gitlab.pot.

The strings stay wrapped in the existing s__ i18n helper, so they remain translatable.

References

• #629580 (closed)

Screenshots or screen recordings

This is a one-word copy change to an existing success alert message ("cancelled" to "canceled"). There is no visual layout change.

Before	After

How to set up and validate locally

1. Run the affected Jest specs:

   node_modules/.bin/jest --config jest.config.js \
     ee/spec/frontend/ai/duo_agents_platform/pages/show/duo_agents_platform_show_spec.js \
     ee/spec/frontend/ai/duo_agents_platform/components/common/agent_flow_user_approval_spec.js

2. Confirm the updated specs pass and the alert message reads "Session has been canceled successfully."

MR acceptance checklist

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