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/federation.module.ts

@@ -5,10 +5,11 @@ import { EnrollmentController } from './enrollment.controller.js';
 import { EnrollmentService } from './enrollment.service.js';
 import { FederationController } from './federation.controller.js';
 import { GrantsService } from './grants.service.js';
+import { FederationClientService } from './client/index.js';
 
 @Module({
   controllers: [EnrollmentController, FederationController],
-  providers: [AdminGuard, CaService, EnrollmentService, GrantsService],
-  exports: [CaService, EnrollmentService, GrantsService],
+  providers: [AdminGuard, CaService, EnrollmentService, GrantsService, FederationClientService],
+  exports: [CaService, EnrollmentService, GrantsService, FederationClientService],
 })
 export class FederationModule {}