test(groot): regression coverage and CI guards for the N1.7 review fixes

Adds/updates unit tests for the N1.5 removal surfaces (config, checkpoint
markers, removed processor steps, v2 action-unpack registration), the
legacy-default remap warnings, action_decode_transform auto/none resolution,
2-D action decoding, the per-instance raw-state cache and pack/decode
reconnection, raw-checkpoint stats fallback and override handling, camera-match
warnings, bf16 handling, and backbone loading kwargs. Adds pytest.importorskip
guards so the fast_tests tiers pass without transformers, and asserts the
training forward pass returns a finite loss.

Note: these tests exercise symbols introduced by the GR00T N1.7 source PRs
(source-core, backbone); merge those for green CI on this branch.

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

tests/policies/groot/test_groot_lerobot.py

@@ -207,6 +207,11 @@ def test_lerobot_groot_forward_pass():
     with torch.no_grad():
         lerobot_loss, lerobot_metrics = lerobot_policy.forward(batch_lerobot_processed)
 
+    assert isinstance(lerobot_loss, torch.Tensor)
+    assert torch.isfinite(lerobot_loss).all()
+    assert "loss" in lerobot_metrics
+    assert np.isfinite(lerobot_metrics["loss"])
+
     print("\nForward pass successful.")
     print(f"  - Loss: {lerobot_loss.item():.6f}")
     print(f"  - Metrics: {lerobot_metrics}")