[P2-E2.6] Wallet API and scan-proxy: buy traffic for a registered synchronizer

Path B of the buy path, split out of #39 which is now scoped to the validator auto-top-up path only.

The end-user wallet path cannot carry a registration either: `BuyTrafficRequest.daml:62` hardcodes `optRegisteredSynchronizer = None`, `createBuyTrafficRequest` in `wallet-external.yaml` has no registration field, and `HttpScanProxyHandler` proxies 15 operations, none registration-related.

**Decide whether this is in Phase 2 before starting.** If it is out, put an explicit descope note on #39 so the gap is recorded rather than implied.

**Key files:** `daml/splice-wallet/daml/Splice/Wallet/BuyTrafficRequest.daml`, `apps/wallet/src/main/openapi/wallet-external.yaml`, `apps/validator/src/main/scala/.../HttpScanProxyHandler.scala`

**Acceptance criteria:**
- [ ] A wallet user can request traffic for a registered synchronizer, with the registration supplied via explicit disclosure
- [ ] The scan-proxy exposes whatever registration lookup the wallet frontend needs

**Depends on:** #39

**Epic:** ChainSafe/canton-extending-mainnet#65