fix(langgraph): satisfy no-empty-function on empty-stream test mock

The construction-only test stub used an empty async generator
`(async function* () {})()` which @typescript-eslint/no-empty-function
flags as an error, breaking `langgraph:lint` in the publish path (which
lints fresh via --skip-nx-cache). Document the empty body so the rule
passes; behaviour is unchanged (still yields no events).

Surfaced by a release dry-run; not caught at merge because ci.yml lint
is not a required status check.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: blove

libs/langgraph/src/lib/agent.fn.spec.ts

@@ -918,7 +918,9 @@ describe('agent — LANGGRAPH_CLIENT_OPTIONS resolution (no mock transport)', ()
       search: vi.fn().mockResolvedValue([]),
     },
     runs: {
-      stream: vi.fn().mockReturnValue((async function* () {})()),
+      stream: vi.fn().mockReturnValue((async function* () {
+        /* empty stream — construction-only tests assert wiring, not events */
+      })()),
     },
   };