Fall back to the collection account only on infrastructure failures

[TaprootFreak]

Follow-up to #4580. The collection-account fallback there caught issuance failures with .catch(() => null), which is too broad.

Problem

Personal Frick IBAN issuance re-reads and merge-resolves the account owner under its own lock and raises KYC_REQUIRED authoritatively against that fresh owner. The blanket .catch(() => null) swallowed that rejection, and the fallback then re-checked only the request's own selector.userData KYC snapshot. When that snapshot is stale relative to the locked check — an in-flight KYC downgrade, or a merge whose target owner sits below level 50 — a customer the authoritative check had just rejected could still be handed a usable collection account with a valid reference. The same blanket catch also swallowed CURRENCY_UNSUPPORTED and other business rejections.

Change

Tell the two failure kinds apart. An infrastructure failure (the provider is down) is what the fallback exists for and still degrades to null. A business rejection is a BadRequestException — above all KYC_REQUIRED — and is re-thrown so it reaches the caller. Applies to both the explicit and the implicit Frick issuance paths, via a small shared helper.

Verification

A new test covers the discrepancy case: issuance rejects with KYC_REQUIRED while the request snapshot is level 50 → the request rejects with KYC_REQUIRED and the fallback (getBank) is never reached. tsc, ESLint, Prettier clean; buy.service.spec.ts 49 tests green locally.

[TaprootFreak]

Two review passes to reach zero findings.

• Pass 1 confirmed the classification is correct (infrastructure failures degrade to the collection account, business rejections propagate) and proved by counter-test that the guard now closes the gap. It flagged that only the implicit path had a test for a real business rejection, and a comment sat above the wrong method.
• Pass 2 (fixes): added the same business-rejection test on the explicit path and moved the helper so each comment sits above its method. Clean on both tracks.

All checks green; buy.service.spec 50 tests green.