fix: Add RollingUpdateSpec validation and deploying_revision_id null check

- Add Pydantic model_validator to reject max_surge=0 + max_unavailable=0
  (deadlock scenario where no progress is possible)
- Add explicit ValueError when deploying_revision_id is None in
  evaluate_cycle to fix mypy arg-type error
- Update test to expect ValueError for None deploying_revision_id

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jopemachine

src/ai/backend/manager/models/deployment_policy/row.py

@@ -6,7 +6,7 @@
 from typing import TYPE_CHECKING
 
 import sqlalchemy as sa
-from pydantic import BaseModel
+from pydantic import BaseModel, model_validator
 from sqlalchemy.dialects import postgresql as pgsql
 from sqlalchemy.orm import Mapped, foreign, mapped_column, relationship
 
@@ -38,6 +38,12 @@ class RollingUpdateSpec(BaseModel):
     max_surge: int = 1
     max_unavailable: int = 0
 
+    @model_validator(mode="after")
+    def _check_not_both_zero(self) -> RollingUpdateSpec:
+        if self.max_surge == 0 and self.max_unavailable == 0:
+            raise ValueError("max_surge or max_unavailable must be positive")
+        return self
+
 
 class BlueGreenSpec(BaseModel):
     """Specification for blue-green deployment strategy."""

src/ai/backend/manager/sokovan/deployment/strategy/rolling_update.py

@@ -69,7 +69,10 @@ def evaluate_cycle(
             5. Compute allowed surge/unavailable, decide create/terminate -> PROGRESSING.
         """
         desired = deployment.replica_spec.target_replica_count
-        classified = self._classify_routes(routes, deployment.deploying_revision_id)
+        deploying_revision_id = deployment.deploying_revision_id
+        if deploying_revision_id is None:
+            raise ValueError("deploying_revision_id must not be None for rolling update")
+        classified = self._classify_routes(routes, deploying_revision_id)
 
         if result := self._check_provisioning(deployment, classified):
             return result

tests/unit/manager/sokovan/deployment/strategy/test_rolling_update.py

@@ -724,17 +724,14 @@ def test_large_desired_surge_1_unavailable_0_creates_exactly_1(self) -> None:
         assert _count_rollout(result) == 1
         assert len(_drain_ids(result)) == 0
 
-    def test_deploying_rev_none_all_routes_classified_as_old(self) -> None:
-        """If deploying_revision_id is None, all routes are old (r.revision_id != None)."""
+    def test_deploying_rev_none_rejected(self) -> None:
+        """If deploying_revision_id is None, evaluate_cycle raises ValueError."""
         deployment = make_deployment(desired=1, deploying_revision_id=None)  # type: ignore[arg-type]
         spec = RollingUpdateSpec(max_surge=1, max_unavailable=0)
         routes = [make_route(revision_id=OLD_REV, status=RouteStatus.HEALTHY)]
 
-        result = RollingUpdateStrategy(spec).evaluate_cycle(deployment, routes)
-
-        # All classified as old, no new → PROGRESSING with create
-        assert result.sub_step == DeploymentSubStep.PROGRESSING
-        assert _count_rollout(result) == 1
+        with pytest.raises(ValueError, match="deploying_revision_id must not be None"):
+            RollingUpdateStrategy(spec).evaluate_cycle(deployment, routes)
 
     def test_route_without_revision_classified_as_old(self) -> None:
         """Routes with revision_id=None are classified as old."""