test(e2e): rename registry entry to euroc_machine_hall with real SHA256

The prior registry entry was speculative: ``euroc_mh01`` pointing at an
old ``robotics.ethz.ch`` URL that no longer resolves (TCP timeout).
The dataset moved to ETH Research Collection (DOI 10.3929/ethz-b-000690084)
as a single 12.6 GB ``machine_hall.zip`` bundle containing MH_01…MH_05.
There's no stable direct download URL — DSpace gates behind a UI —
so:

- Renamed entry: ``euroc_mh01`` → ``euroc_machine_hall`` (matches the
  actual artifact).
- SHA256 set to the real bundle hash 5ed7d07…
- URL left empty (same pattern as ``vpair_sample``); the CLI now
  exits 3 and prints fetch instructions for empty-URL entries instead
  of crashing on ``urllib.request.urlretrieve("")``.
- Adapter ``DatasetNotAvailableError`` message and conftest skip-reason
  updated to tell engineers how to fetch/unpack manually.
- ``test_registry_has_euroc_machine_hall`` pin test replaces the old
  pin; asserts real hash (not the ``"0"*64`` placeholder).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/e2e/test_download.py

@@ -11,12 +11,15 @@ from gps_denied.testing.download import (
 )
 
 
-def test_registry_has_euroc():
-    assert "euroc_mh01" in DATASET_REGISTRY
-    spec = DATASET_REGISTRY["euroc_mh01"]
+def test_registry_has_euroc_machine_hall():
+    assert "euroc_machine_hall" in DATASET_REGISTRY
+    spec = DATASET_REGISTRY["euroc_machine_hall"]
     assert isinstance(spec, DatasetSpec)
-    assert spec.url.startswith("http")
+    # URL intentionally empty — ETH Research Collection gates downloads behind
+    # a DSpace UI without a stable direct URL. Registry records SHA256 only.
+    assert spec.url == ""
     assert len(spec.sha256) == 64
+    assert spec.sha256 != "0" * 64  # real hash, not the placeholder
 
 
 def test_registry_has_vpair_sample():