docs(BA-6041): deprecate MountOption.mount_destination for session creation paths

This field originally entered the per-vfolder option model in PR #2168
(service-creation wire schema only). Session creation never read it —
mount_map / mount_id_map carry destinations there. Mark it deprecated and
state explicitly that it applies to inference service creation only, so
callers stop putting it under session mount_options where it is silently
ignored.

Follow-up to BA-6040 / PR #11608.

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

Author: jopemachine

src/ai/backend/common/dto/manager/session/types.py

@@ -154,7 +154,17 @@ class MountOption(BaseFieldModel):
 
     mount_destination: str | None = Field(
         default=None,
-        description="Mount destination inside the container. Defaults to ``/home/work/{folder_name}``.",
+        deprecated=(
+            "Inline mount_destination is supported only by the inference service creation"
+            " wire schema (``ServiceConfigModel.extra_mounts``) for legacy reasons. Session"
+            " creation (``CreationConfigV*.mount_options``) does NOT read this field — use"
+            " ``mount_map`` / ``mount_id_map`` to specify destinations for sessions."
+        ),
+        description=(
+            "Mount destination inside the container. Defaults to ``/home/work/{folder_name}``."
+            " **Service creation only** — ignored on session creation paths; supply session"
+            " destinations via ``mount_map`` / ``mount_id_map`` instead."
+        ),
     )
     type: MountTypes = MountTypes.BIND
     permission: MountPermission | None = Field(