fix(envs): set LiberoEnvConfig.fps default to 20 to match robosuite (#4124)

* fix(envs): set LiberoEnvConfig.fps default to 20 to match robosuite

LiberoEnvConfig.fps was set to 30, but the underlying robosuite
OffScreenRenderEnv always runs at its default control_freq of 20 Hz
since fps is never passed through. This mismatch silently decouples
the dataset/eval loop rate from the actual simulation step rate.

Set the default to 20 to match the real sim rate and avoid the
footgun.

Fixes #3368

* fix(libero): apply configured control frequency

---------

Co-authored-by: xinmotlanthua <275663218+xinmotlanthua@users.noreply.github.com>

tests/envs/test_dispatch.py

@@ -35,6 +35,17 @@ def test_unknown_type():
         make_env_config("nonexistent")
 
 
+def test_libero_fps_controls_simulator_frequency():
+    cfg = LiberoEnv(fps=17)
+
+    assert cfg.gym_kwargs["control_freq"] == 17
+
+
+def test_libero_rejects_nonpositive_fps():
+    with pytest.raises(ValueError, match="fps must be positive"):
+        LiberoEnv(fps=0)
+
+
 def test_identity_processors():
     """Base class get_env_processors() returns identity pipelines."""
     cfg = make_env_config("aloha")