Add DMTF mockup fixtures and test helpers

Unmodified files from DMTF Redfish-Publications (public-rackmount1, public-localstorage, public-telemetry), only those the tests use, each set with DMTF's BSD-3-Clause license and a COMMANDS.txt naming the source commit and request paths.

conftest.py loads them by request path and serves a mockup over mocked HTTP, with a SessionService that hands out a token, for the transport tests.

tests/fixtures/dmtf-public-rackmount1/Managers/BMC/EthernetInterfaces/index.json

@@ -0,0 +1,16 @@
+{
+    "@odata.type": "#EthernetInterfaceCollection.EthernetInterfaceCollection",
+    "Name": "Ethernet Network Interface Collection",
+    "Description": "Collection of EthernetInterfaces for this Manager",
+    "Members@odata.count": 2,
+    "Members": [
+        {
+            "@odata.id": "/redfish/v1/Managers/BMC/EthernetInterfaces/eth0"
+        },
+        {
+            "@odata.id": "/redfish/v1/Managers/BMC/EthernetInterfaces/ToHost"
+        }
+    ],
+    "@odata.id": "/redfish/v1/Managers/BMC/EthernetInterfaces",
+    "@Redfish.Copyright": "Copyright 2014-2026 DMTF. For the full DMTF copyright policy, see http://www.dmtf.org/about/policies/copyright."
+}