fix(billing): Guard against missing plan details in subscription header

[sentry]

This PR fixes a TypeError: undefined is not an object (evaluating 't.planDetails.name') that occurred in the /settings/billing/ page. The error happened when the SubscriptionHeader component attempted to access subscription.planDetails.name, but planDetails was undefined. This can occur for users whose accounts are disabled due to seat limits, as the API (/customers/{orgSlug}/) returns a minimal subscription object via DisabledMemberCustomerSerializer that omits planDetails.

The fix involves:

• Conditionally rendering the plan name and icon block in SubscriptionHeader only when subscription.planDetails is present.
• Guarding the getPlanIcon call with a check for subscription.planDetails to prevent crashes if it's missing.

The TypeScript Subscription type was intentionally left as planDetails: Plan (required) to avoid introducing widespread type errors across other components that correctly assume planDetails is always present for active subscriptions.

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Fixes JAVASCRIPT-3BS1

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