fix(billing): translate stripe errors in billing services

[whoAbhishekSah]

What

Wraps every stripe call in the billing services with TranslateStripeError from #1849, so a typed provider error leaves the service instead of a raw stripe error. Touches the customer, subscription, checkout, product, and invoice services — all 30 stripe call sites.

Along the way:

• The old err.(*stripe.Error) type assertions become errors.Is / errors.As, which also match wrapped errors. The type assertion silently failed on any error that had been wrapped.
• The product and invoice calls that returned bare errors now carry context ("failed to create price at billing provider: ...").
• ListPaymentMethods never checked the stripe iterator's error, so a failed list looked like an empty list. It now returns the error.

Behavior kept as before: a customer already deleted on Stripe is still tolerated during delete, and a missing subscription still maps to ErrSubscriptionOnProviderNotFound.

Part of #1836. Stack: translator (#1849) → this PR → handler error mapping.

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated (UTC)
frontier	Ready	Preview	Aug 5, 2026 11:42am

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[coveralls]

Coverage Report for CI Build 30974219068

Coverage increased (+0.005%) to 47.564%

Details

• Coverage increased (+0.005%) from the base build.
• Patch coverage: 44 uncovered changes across 5 files (3 of 47 lines covered, 6.38%).
• 2 coverage regressions across 1 file.

Uncovered Changes

File	Changed	Covered	%
billing/customer/service.go	15	3	20.0%
billing/invoice/service.go	10	0	0.0%
billing/subscription/service.go	10	0	0.0%
billing/checkout/service.go	7	0	0.0%
billing/product/service.go	5	0	0.0%

Coverage Regressions

2 previously-covered lines in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
billing/customer/service.go	2	31.87%

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Coverage Stats

Relevant Lines:	39381
Covered Lines:	18731
Line Coverage:	47.56%
Coverage Strength:	15.4 hits per line

---

💛 - Coveralls

[rohilsurana]

I verified the coverage claim on this branch: 32 stripeClient call sites across the five services, and every one either wraps its error with TranslateStripeError or reaches it through an iterator .Err() check that wraps. No err.(*stripe.Error) type assertions are left under billing/.

The two tolerated cases keep their old behavior: a customer already deleted on Stripe is still tolerated during Delete, and a missing subscription still becomes ErrSubscriptionOnProviderNotFound. The ErrorCodeInvoiceUpcomingNone special case in GetUpcoming is also preserved.

The ListPaymentMethods fix is a real bug fix. A failed list used to look like an empty list.

Two small comments inline.

[rohilsurana]

LGTM, verified all stripe call sites are covered