test(BA-6041): MountOption.mount_destination is dropped on session creation but preserved on service creation

Two unit tests pinning the asymmetry the deprecation message describes:

- ``AgentRegistry._mount_entries_from_creation_config`` ignores mount_destination
  inside ``mount_options``; only ``mount_id_map`` can supply destinations for
  session creation.
- ``manager.data.model_serving.types.MountOption.from_dto`` carries mount_destination
  through to the data layer for the inference service creation flow.

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

Author: jopemachine

tests/unit/manager/test_registry.py

@@ -18,6 +18,7 @@
     ModelHealthCheck,
     ModelServiceConfig,
 )
+from ai.backend.common.dto.manager.session.types import MountOption as MountOptionDTO
 from ai.backend.common.events.event_types.session.broadcast import SchedulingBroadcastEvent
 from ai.backend.common.events.types import AbstractEvent
 from ai.backend.common.plugin.hook import HookPluginContext
@@ -28,6 +29,7 @@
     SlotName,
 )
 from ai.backend.manager.config.provider import ManagerConfigProvider
+from ai.backend.manager.data.model_serving.types import MountOption
 from ai.backend.manager.data.session.types import SessionStatus
 from ai.backend.manager.errors.kernel import SessionNotFound
 from ai.backend.manager.plugin.network import NetworkPluginContext
@@ -392,3 +394,40 @@ async def _hang(cache_id: str) -> AsyncIterator[AbstractEvent]:
                 await mock_registry_obj._wait_for_session_running(
                     session_id, mock_propagator, max_wait=0
                 )
+
+
+class TestMountDestinationAsymmetry:
+    """``MountOption.mount_destination`` is honored only on the inference service
+    creation path; session creation silently drops it from ``mount_options`` and
+    pulls destinations from ``mount_id_map`` instead. This is why the field is
+    marked deprecated for session callers (see common/dto/manager/session/types.py).
+    """
+
+    def test_session_creation_drops_mount_destination_from_mount_options(self) -> None:
+        vfid = uuid.uuid4()
+        creation_config = {
+            "mount_ids": [vfid],
+            "mount_id_map": {},  # caller did not provide a destination here
+            "mount_options": {
+                vfid: {"mount_destination": "/should/be/ignored", "permission": "ro"},
+            },
+        }
+
+        entries = AgentRegistry._mount_entries_from_creation_config(creation_config)
+
+        assert len(entries) == 1
+        # mount_destination from mount_options is silently dropped — only
+        # mount_id_map can supply destinations for session creation.
+        assert entries[0].mount_destination is None
+
+    def test_inference_service_preserves_mount_destination_via_from_dto(self) -> None:
+        dto = MountOptionDTO.model_validate({
+            "mount_destination": "/models/extra",
+            "perm": "ro",
+        })
+
+        data = MountOption.from_dto(dto)
+
+        # Service-creation flow carries mount_destination through to the data
+        # layer (and ultimately to VFolderMount.kernel_path).
+        assert data.mount_destination == "/models/extra"