test: stabilize gateway server shard (#77131)

test/vitest/vitest.gateway-server.config.ts

@@ -20,6 +20,9 @@ export function createGatewayServerVitestConfig(env?: Record<string, string | un
         "src/gateway/server.startup-matrix-migration.integration.test.ts",
         "src/gateway/sessions-history-http.test.ts",
       ],
+      // Gateway server suites share process-level env, logger, and server helper state.
+      // Isolate files so parallel shards cannot cross-wire suite-scoped servers.
+      isolate: true,
       name: "gateway-server",
     },
   );