fix(BA-5982): replace ValueError with RevisionMissingModelVFolder (BA-400)

``DeploymentSessionDraftBuilder.build`` and ``_resolve_mounts`` were
raising bare ``ValueError`` when ``model_mount_config.vfolder_id`` is
``None`` — a real reachable state, since ``deployment_revisions.model``
is a SET NULL FK that collapses to NULL when the backing vfolder row is
deleted. Per the project rule, business-logic exceptions must inherit
from ``BackendAIError``.

Add ``RevisionMissingModelVFolder(BackendAIError, web.HTTPBadRequest)``
to ``manager/errors/deployment.py`` and switch both raise sites in
``deployment_draft_builder.py`` to use it. Other call sites that detect
the same condition (``SessionValidationSpec.from_revision_spec`` /
``from_revision``, ``service._build_creator_from_revision_data``)
already raise ``InvalidAPIParameters`` so they stay
``BackendAIError``-based and unchanged.

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

Author: jopemachine

src/ai/backend/manager/errors/deployment.py

@@ -66,6 +66,29 @@ def error_code(self) -> ErrorCode:
         )
 
 
+class RevisionMissingModelVFolder(BackendAIError, web.HTTPBadRequest):
+    """A revision's model vfolder reference is null.
+
+    Raised when the draft / session pipeline reads a
+    ``ModelRevisionData`` whose ``model_mount_config.vfolder_id`` has
+    collapsed to ``NULL`` because the backing vfolder row was deleted
+    (``vfolders.id`` SET NULL FK on ``deployment_revisions.model``).
+    The revision is preserved for history, but no new session can be
+    spawned from it until a new revision pointing at a live model
+    vfolder takes over.
+    """
+
+    error_type = "https://api.backend.ai/probs/revision-missing-model-vfolder"
+    error_title = "Deployment revision has no model vfolder."
+
+    def error_code(self) -> ErrorCode:
+        return ErrorCode(
+            domain=ErrorDomain.MODEL_SERVICE,
+            operation=ErrorOperation.READ,
+            error_detail=ErrorDetail.INVALID_PARAMETERS,
+        )
+
+
 class RevisionNotDeployable(BackendAIError, web.HTTPConflict):
     """A revision references resources that no longer exist.
 

src/ai/backend/manager/sokovan/deployment/deployment_draft_builder.py

@@ -44,6 +44,7 @@
     ResourceOpts,
     SessionHandlerOptions,
 )
+from ai.backend.manager.errors.deployment import RevisionMissingModelVFolder
 from ai.backend.manager.repositories.scheduler.types.session_creation import DeploymentContext
 
 
@@ -76,7 +77,7 @@ def build(
         model_definition_payload = cls._model_definition_payload(target_revision, context)
 
         if target_revision.model_mount_config.vfolder_id is None:
-            raise ValueError(
+            raise RevisionMissingModelVFolder(
                 f"Revision {target_revision.id} has no model vfolder; cannot build session draft"
             )
 
@@ -152,7 +153,7 @@ def _resolve_mounts(
         # Model vfolder is always first (READ_ONLY), extra mounts follow
         # with their frozen permissions already on each entry.
         if target_revision.model_mount_config.vfolder_id is None:
-            raise ValueError(
+            raise RevisionMissingModelVFolder(
                 f"Revision {target_revision.id} has no model vfolder; cannot build mount entries"
             )
         return (