fix(payments-ui): Fix circular dependency in assertCartOwnership decorator

[xlisachan]

Because

• The static import of getSessionUid from @fxa/payments/ui-auth in assertCartOwnership.decorator.ts created a circular dependency: decorator → ui-auth → ui/server → app.ts → nextjs-actions.service → decorator.
• This caused a TDZ ReferenceError ("Cannot access 'NextJSActionsService' before initialization") on stage, returning 500 for all SSR pages including /subscriptions/manage.

This pull request

• Changes the @fxa/payments/ui-auth import in the decorator to a dynamic await import(), breaking the circular module evaluation chain.
• Adds unit tests for the AssertCartOwnership decorator.

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