fix(apple-iap): fall back to cached IAP records on App Store failures

[david1alvarez]

Because:

• The App Store Server API intermittently fails, and app-store-server-api surfaces those as an AppStoreError carrying one of Apple's 5xxxxxx codes.
• AppleIapPurchaseManager.getForUser rethrew everything but its not-found cases, so an Apple outage failed EligibilityService.checkEligibility and took cart setup and checkout down with it.
• Treating a purchase we cannot verify as absent is worse than erroring: a customer with an active Apple subscription could buy an overlapping web subscription.

This commit:

• Adds AppleIapServiceUnavailableError, and maps Apple's server-range error codes plus the library's isRetryable and isRateLimitExceeded flags to it in AppleIapClient.convertError.
• Adds AppleIapPurchaseManager.getStaleCachedForUser, which reads a customer's purchase records from Firestore without refreshing them against Apple.
• Falls back to those records in EligibilityService.checkEligibility, so a purchase we cannot verify still blocks an overlapping web purchase.
• Exports the Apple IAP error module from the payments-iap barrel, and passes the logger to EligibilityService in initSubplat.
• Leaves the billing endpoint and the subscription management page erroring, so a customer sees a failure rather than an empty subscription list.

Closes PAY-3841

Checklist

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[Copilot]

Pull request overview

This PR improves resiliency of Apple IAP eligibility checks during intermittent App Store Server API outages by classifying Apple-side failures as a distinct error and falling back to cached Firestore purchase records for overlap protection (so an unverified Apple subscription doesn’t appear “absent” and allow an overlapping web purchase).

Changes:

• Introduces AppleIapServiceUnavailableError and maps Apple server-range error codes (5xxxxxx) plus app-store-server-api retry/rate-limit signals to it in AppleIapClient.
• Adds AppleIapPurchaseManager.getStaleCachedForUser to read cached Apple purchase records from Firestore without calling Apple.
• Updates EligibilityService.checkEligibility to fall back to cached Apple purchases when Apple is unavailable, and wires a logger through initSubplat; adds unit test coverage for the new behaviors.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/fxa-auth-server/lib/payments/initSubplat.ts	Passes a logger into EligibilityService initialization to support new warning logging on Apple outages.
libs/payments/iap/src/lib/apple/apple-iap.error.ts	Adds AppleIapServiceUnavailableError to represent Apple-side/transient failures with structured info.
libs/payments/iap/src/lib/apple/apple-iap.client.ts	Detects Apple-side/transient App Store errors and converts them to AppleIapServiceUnavailableError.
libs/payments/iap/src/lib/apple/apple-iap.client.spec.ts	Adds tests ensuring retryable/server/rate-limit App Store errors map to AppleIapServiceUnavailableError and non-retryable client errors do not.
libs/payments/iap/src/lib/apple/apple-iap-purchase.manager.ts	Adds getStaleCachedForUser to return Firestore-cached purchases without contacting Apple.
libs/payments/iap/src/lib/apple/apple-iap-purchase.manager.spec.ts	Adds tests for getStaleCachedForUser and verifies AppleIapServiceUnavailableError is propagated to callers.
libs/payments/iap/src/index.ts	Exports Apple IAP error types from the library barrel for cross-package consumption.
libs/payments/eligibility/src/lib/eligibility.service.ts	Falls back to cached Apple purchases when Apple is unavailable (and logs a warning), preventing overlapping web purchases from slipping through.
libs/payments/eligibility/src/lib/eligibility.service.spec.ts	Adds tests covering the fallback-to-cache behavior and confirming non-retryable Apple errors are rethrown.