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/Systems/437XR1138R2/Memory/DIMM4/index.json

@@ -0,0 +1,23 @@
+{
+    "@odata.type": "#Memory.v1_24_0.Memory",
+    "Id": "DIMM4",
+    "Name": "DIMM Slot 4",
+    "MemoryLocation": {
+        "Socket": 2,
+        "MemoryController": 2,
+        "Channel": 1,
+        "Slot": 4
+    },
+    "Location": {
+        "PartLocation": {
+            "ServiceLabel": "DIMM 4",
+            "LocationType": "Slot",
+            "LocationOrdinalValue": 3
+        }
+    },
+    "Status": {
+        "State": "Absent"
+    },
+    "@odata.id": "/redfish/v1/Systems/437XR1138R2/Memory/DIMM4",
+    "@Redfish.Copyright": "Copyright 2014-2026 DMTF. For the full DMTF copyright policy, see http://www.dmtf.org/about/policies/copyright."
+}