fix(billing): refine retryWithBackoff — short-circuit on non-transient Stripe errors + restore err.stack

[PierreBrisorgueil]

Context

Phase 2 PR #3690 (refund correlation backfill retry) ships retryWithBackoff that retries on ALL error types. Independent /critical-review fallback flagged 1 medium + 1 low for a future polish pass.

Findings

Medium — Blind retry on non-transient Stripe errors

modules/billing/lib/billing.retry.js:17-29 — retryWithBackoff has no error filter; retries on invalid_request_error, idempotency_error just as eagerly as on 500s.

• invalid_request_error (bad paymentIntentId, auth failure) : deterministic failure → wastes 600ms before dead-letter
• idempotency_error (duplicate request key) : retry may produce different result but amplifies webhook handler latency

Fix: short-circuit on err.type === 'invalid_request_error' (StripeInvalidRequestError) — skip retry, go straight to dead-letter.

Low — err.stack dropped in new error log

modules/billing/services/billing.webhook.service.js:331 — the replaced logger.warn included stack: err?.stack ; the new logger.error (more severe!) omits it.

Inconsistent with other logger.error calls on evtErr in same file (lines 1021, 1054, 1214, 403, 438) which all include evtErr?.stack.

Fix: add stack: err?.stack to the meta object on both new logger.error calls (outer + inner DLQ).

Acceptance

• Retry loop short-circuits on non-transient Stripe error types
• err.stack included on both new logger.error calls (PI backfill exhaustion + DLQ write failure)
• Unit tests cover the short-circuit path (assert 1 call, not 3, on invalid_request_error)

Source

Discovered by /critical-review Claude fallback on PR #3690 (CR rate-limited 2026-05-21).

[PierreBrisorgueil]

Update: the err.stack restore + consistent stripeSessionId key part landed directly in PR #3690 (commit 0c5ac16) while resolving CodeRabbit/Copilot threads.

Remaining scope for this issue is now ONLY:

• Short-circuit retryWithBackoff on non-transient Stripe errors (err.type === 'invalid_request_error') — skip retry, go straight to dead-letter.

The err.stack + key-naming items are done.