fix(billing): Guard against undefined planDetails in SubscriptionHeader

[sentry]

This PR addresses a TypeError: Cannot read properties of undefined (reading 'name') that occurred on the /settings/billing/ page when subscription.planDetails was undefined.

The root cause was the SubscriptionHeader component attempting to access subscription.planDetails.name without a null guard, which happened for organizations without an active paid plan or in a transitional billing state.

Changes include:

• In static/gsApp/views/subscriptionPage/subscriptionHeader.tsx:
  • Added a conditional check before calling getPlanIcon to ensure subscription.planDetails is defined.
  • Implemented optional chaining (subscription.planDetails?.name) and a fallback text (t('Subscription')) for the plan name display.
• In static/gsApp/utils/billing.tsx:
  • Updated the supportsPayg function to use optional chaining (subscription.planDetails?.allowOnDemand) to prevent a similar crash when planDetails is undefined.
• In static/gsApp/views/subscriptionPage/subscriptionHeader.spec.tsx:
  • Added a new test case to verify that the SubscriptionHeader component renders correctly with fallback text when planDetails is undefined.

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Fixes JAVASCRIPT-3BPY

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