Implements FederationClientService — a NestJS injectable that dials peer
gateways over mTLS (undici Agent with cert+sealed-key from federation_peers),
invokes list/get/capabilities verbs, validates responses via Zod, and surfaces
all failure modes as typed FederationClientError with a coherent error code
taxonomy (PEER_NOT_FOUND, PEER_INACTIVE, PEER_MISCONFIGURED, NETWORK,
FORBIDDEN, HTTP_{status}, INVALID_RESPONSE).
Per-peer Agent instances are cached in a Map for the service lifetime;
flushPeer(peerId) invalidates the cache for M5/M6 cert rotation and
revocation events.
Wired into FederationModule providers + exports so QuerySourceService
(M3-09) can inject it.
13 unit tests covering all required scenarios via undici MockAgent +
real sealClientKey/unsealClientKey round-trip.
Closes#462
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
