refactor(BA-5936): make CascadeChild generic over its row type

Parameterize CascadeChild on the cascade table's Row class
(``CascadeChild[TRow: Base]``) for consistency with PrunerSpec[TRow]
and to give subclass authors precise typing on ``row_class()`` —
e.g., ``CascadeChild[KernelRow]`` makes ``row_class()`` return
``type[KernelRow]`` instead of the over-broad ``type[Base]``.

PrunerSpec.cascade is typed as ``list[CascadeChild[Any]]`` since a
spec composes cascades over heterogeneous tables.

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

Author: fregataa

src/ai/backend/manager/repositories/base/pruner.py

@@ -24,7 +24,7 @@
 TRow = TypeVar("TRow", bound=Base)
 
 
-class CascadeChild(ABC):
+class CascadeChild[TRow: Base](ABC):
     """A child table whose rows must be deleted before the parent's prune.
 
     Used for simple FK cascades. ``execute_pruner`` first locks and
@@ -36,13 +36,16 @@ class CascadeChild(ABC):
     where ``<target_ids>`` is the list returned from the single
     ``SELECT pk FOR UPDATE`` against the parent table.
 
+    The type parameter ``TRow`` is the cascade table's ORM Row class —
+    e.g., ``CascadeChild[KernelRow]``.
+
     Polymorphic / cross-cutting cleanups (e.g., RBAC associations) are not
     handled here — see :meth:`PrunerSpec.entity_type` for that.
     """
 
     @classmethod
     @abstractmethod
-    def row_class(cls) -> type[Base]:
+    def row_class(cls) -> type[TRow]:
         """ORM Row class for the cascade table.
 
         Example:
@@ -89,7 +92,7 @@ class PrunerSpec[TRow: Base](ABC):
     """
 
     conditions: list[QueryCondition] = field(default_factory=list)
-    cascade: list[CascadeChild] = field(default_factory=list)
+    cascade: list[CascadeChild[Any]] = field(default_factory=list)
     limit: int = DEFAULT_PRUNE_LIMIT
     cascade_rbac: bool = True
 

tests/unit/manager/repositories/base/test_pruner.py

@@ -60,9 +60,9 @@ class PrunerTestChildRow(Base):  # type: ignore[misc]
     name = sa.Column(sa.String(50), nullable=False)
 
 
-class TestChildCascade(CascadeChild):
+class TestChildCascade(CascadeChild[PrunerTestChildRow]):
     @classmethod
-    def row_class(cls) -> type[Base]:
+    def row_class(cls) -> type[PrunerTestChildRow]:
         return PrunerTestChildRow
 
     @classmethod