feat(BA-6054): expose subpath on v2 REST/GraphQL session MountItemInput

Follow-up to BA-6040 (#11608). Add `subpath` to the v2 `MountItemInput`
DTO so v2 REST and GraphQL session-creation callers can mount a vfolder
subpath via `EnqueueSessionInput.mounts`. Propagate the field through
the session adapter and the kernel execution spec adapter to
`MountInfoEntry`, and regenerate the v2/supergraph GraphQL schemas.

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

Author: jopemachine

docs/manager/graphql-reference/supergraph.graphql

@@ -17112,6 +17112,9 @@ input SessionMountItemInput
 
   """Mount permission ('rw' or 'ro')."""
   permission: String = null
+
+  """Subpath within the vfolder to mount. Null mounts the vfolder root."""
+  subpath: String = null
 }
 
 """Added in 25.13.0."""

docs/manager/graphql-reference/v2-schema.graphql

@@ -11636,6 +11636,9 @@ input SessionMountItemInput {
 
   """Mount permission ('rw' or 'ro')."""
   permission: String = null
+
+  """Subpath within the vfolder to mount. Null mounts the vfolder root."""
+  subpath: String = null
 }
 
 """Added in 26.4.2. Additional resource options for session creation."""

src/ai/backend/common/dto/manager/v2/session/request.py

@@ -261,6 +261,14 @@ class MountItemInput(BaseRequestModel):
     permission: str | None = Field(
         default=None, description="Mount permission override ('rw' or 'ro')."
     )
+    subpath: str | None = Field(
+        default=None,
+        min_length=1,
+        description=(
+            "Subpath within the vfolder to mount. Omit (null) to mount the vfolder root."
+            " Empty string is rejected."
+        ),
+    )
 
 
 class BatchConfigInput(BaseRequestModel):

src/ai/backend/manager/api/adapter_options/session/options.py

@@ -200,6 +200,7 @@ def _mount_items_to_entries(items: Sequence[MountItemInput]) -> list[MountInfoEn
                 vfolder_id=VFolderUUID(item.vfolder_id),
                 mount_destination=item.mount_path,
                 mount_perm=perm or MountPermission.READ_WRITE,
+                subpath=item.subpath,
             )
         )
     return result

src/ai/backend/manager/api/adapters/session/adapter.py

@@ -251,6 +251,7 @@ async def enqueue(
                     vfolder_id=VFolderUUID(m.vfolder_id),
                     mount_destination=m.mount_path,
                     mount_perm=(MountPermission(m.permission) if m.permission else None),
+                    subpath=m.subpath,
                 )
                 for m in input.mounts
             ]

src/ai/backend/manager/api/gql/session/types.py

@@ -533,6 +533,10 @@ class SessionMountItemInputGQL(PydanticInputMixin[MountItemInputDTO]):
     vfolder_id: ID = gql_field(description="Virtual folder UUID.")
     mount_path: str | None = gql_field(default=None, description="Custom mount path.")
     permission: str | None = gql_field(default=None, description="Mount permission ('rw' or 'ro').")
+    subpath: str | None = gql_field(
+        default=None,
+        description="Subpath within the vfolder to mount. Null mounts the vfolder root.",
+    )
 
 
 @gql_pydantic_input(