update VLA-JEPA tests for arch changes and action_is_pad

- Switch conftest to use `action_model_type="DiT-test"` now that
  `action_num_heads` / `action_attention_head_dim` have been removed.
- Add action_head tests covering fully-padded loss (zero) and equivalence
  of action_is_pad=None vs all-zeros mask.
- Remove obsolete `test_native_to_lerobot_wm_only` test.

tests/policies/vla_jepa/conftest.py

@@ -70,9 +70,8 @@ def make_config(
         num_embodied_action_tokens_per_instruction=3,
         num_inference_timesteps=2,
         action_hidden_size=QWEN_HIDDEN_SIZE,
+        action_model_type="DiT-test",
         action_num_layers=1,
-        action_num_heads=2,
-        action_attention_head_dim=8,
         predictor_depth=1,
         predictor_num_heads=2,
         predictor_mlp_ratio=2.0,