fix(test): use SyncVectorEnv in test_base_create_envs

AsyncVectorEnv spawns new subprocesses that do not inherit the
in-process gym registration created by the test. Pass
use_async_envs=False since this test validates dispatch logic,
not async parallelism.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/envs/test_dispatch.py

@@ -87,7 +87,7 @@ def test_base_create_envs():
             return {}
 
     try:
-        envs = _Env().create_envs(n_envs=2)
+        envs = _Env().create_envs(n_envs=2, use_async_envs=False)
         assert "_dispatch_base_test" in envs
         env = envs["_dispatch_base_test"][0]
         assert isinstance(env, gym.vector.VectorEnv)