test(federation): fix DB provider injection in M2 E2E test module

Move DB token from overrideProvider (which requires an existing binding)
to the providers array so Nest can resolve GrantsService dependencies
when FederationModule is tested without DatabaseModule.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/gateway/src/__tests__/integration/federation-m2-e2e.integration.test.ts

@@ -86,9 +86,8 @@ describe.skipIf(!stepCaRun)('federation M2 E2E — peer add enrollment flow', ()
 
     const moduleRef = await Test.createTestingModule({
       imports: [FederationModule],
+      providers: [{ provide: DB, useValue: db }],
     })
-      .overrideProvider(DB)
-      .useValue(db)
       .overrideGuard(AdminGuard)
       .useValue({ canActivate: () => true })
       .compile();