feat(federation): outbound mTLS FederationClient (FED-M3-08)

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>

apps/gateway/src/federation/client/index.ts

@@ -0,0 +1,13 @@
+/**
+ * Federation client barrel — re-exports for FederationModule consumers.
+ *
+ * M3-09 (QuerySourceService) and future milestones should import from here,
+ * not directly from the implementation file.
+ */
+
+export {
+  FederationClientService,
+  FederationClientError,
+  type FederationClientErrorCode,
+  type FederationClientErrorOptions,
+} from './federation-client.service.js';