fix: surface response encoding errors instead of bare 500 {}

[ralph-bitgo]

What

• Add customEncodeErrorFormatter in src/shared/errorFormatters.ts: when a
  response payload fails its io-ts codec check in typed-express-router's
  res.sendEncoded(), it logs the real error (message, stack, and request
  context: method, URL, api name) and returns
  { error: 'Internal Server Error', details: <real message> } in the 500
  body — matching the documented 500 ErrorResponse shape.
• Wire encodeErrorFormatter: customEncodeErrorFormatter into every
  createRouter call site alongside the existing decode formatter: the MBE
  API router, both MBE health routers, and the AWM API/health routers.
• Regression tests: an end-to-end test in generateWallet.test.ts makes
  wallet/add return a wallet omitting a required field (as WCN-2395-induced
  responses do) and asserts a descriptive 500 body plus a server-side log
  line; this test fails with the bare 500 {} on pre-fix code. Unit tests in
  errorFormatters.test.ts pin the formatter contract (log content and body
  shape, including the non-Error fallback).

Why

Customers creating type: advanced wallets hit the WCN-2395 permission bug
(creator stuck at view-only because UMS rejects walletType: advanced): the
wallet is created on the backend, but the /wallet/add response then violates
MBE's response schema, and typed-express-router silently swaps the real
validation error for a bare HTTP 500 {} — no error body, no server log,
nothing actionable for the customer or for on-call (originally reported by
AKM as a secondary symptom of WCN-2395). The library's default
encodeErrorFormatter is () => ({}) and MBE only overrode the decode-side
formatter, so the encode side stayed on library defaults.

This is the ticket's immediate/diagnostic fix: every future schema-violating
response becomes debuggable — the client sees which response codec failed and
operators get the full error with stack in the logs — independent of the
upstream WCN-2395 fix. Once WCN-2395 lands upstream, the original symptom
disappears on its own, but this diagnostic layer stays valuable for any
future response-shape regression.

The root fix (UMS enum + wallet-platform retry/DLQ handling) lives in
WCN-2395 and is out of scope for this repo. I deliberately did not relax the
required fields in generateWalletRoute.ts to t.partial: the exact
mismatching field still needs live confirmation, and weakening the declared
contract would mask future server-side bugs.

Test plan

• npm test — full suite: 442 passing, including the new regression test
  (verified it fails with the bare 500 {} against pre-fix wiring) and
  the formatter unit tests
• npm run lint — clean
• npm run build (tsc) — clean
• scripts/check-openapi-masterExpress.sh — OpenAPI spec in sync (no
  route definitions changed)

Ticket: WCN-2396

[linear-code]

WCN-2396