feat(cron-monitors): Add billing seat management to cron monitor detectors

Implements billing seat management for cron monitor detectors via the
MonitorIncidentDetectorValidator. This ensures that cron monitors
created through the detector API properly handle seat assignment,
validation, and removal.

Changes:
- Add validate_enabled() to check seat availability before enabling
- Modify create() to assign seats with graceful degradation when no
  seats are available (detector created but disabled)
- Modify update() to handle enable/disable transitions with seat
  operations and race condition handling
- Add delete() to remove seats immediately when detector is deleted
- Add comprehensive test coverage for all seat management scenarios

Uses generic seat APIs (assign_seat, check_assign_seat, disable_seat,
remove_seat) with DataCategory.MONITOR following the same pattern as
uptime monitors.

Fixes [NEW-619: Ensure CRUD / enable+disable Cron Detectors in new UI handles assigning / unassigning seats](https://linear.app/getsentry/issue/NEW-619/ensure-crud-enabledisable-cron-detectors-in-new-ui-handles-assigning)

Author: evanpurkhiser

src/sentry/monitors/models.py

@@ -50,6 +50,7 @@
 
 if TYPE_CHECKING:
     from sentry.models.project import Project
+    from sentry.workflow_engine.models import Detector
 
 MONITOR_CONFIG = {
     "type": "object",
@@ -812,6 +813,15 @@ class Meta:
         db_table = "sentry_monitorenvbrokendetection"
 
 
+def get_cron_monitor(detector: Detector) -> Monitor:
+    """
+    Given a detector get the matching cron monitor.
+    """
+    data_source = detector.data_sources.first()
+    assert data_source
+    return Monitor.objects.get(id=int(data_source.source_id))
+
+
 @data_source_type_registry.register(DATA_SOURCE_CRON_MONITOR)
 class CronMonitorDataSourceHandler(DataSourceTypeHandler[Monitor]):
     @override

src/sentry/monitors/validators.py

@@ -35,6 +35,7 @@
     MonitorLimitsExceeded,
     ScheduleType,
     check_organization_monitor_limit,
+    get_cron_monitor,
 )
 from sentry.monitors.schedule import get_next_schedule, get_prev_schedule
 from sentry.monitors.types import CrontabSchedule, slugify_monitor_slug
@@ -52,7 +53,7 @@
     BaseDataSourceValidator,
     BaseDetectorTypeValidator,
 )
-from sentry.workflow_engine.models import DataSource, Detector
+from sentry.workflow_engine.models import Detector
 
 MONITOR_STATUSES = {
     "active": ObjectStatus.ACTIVE,
@@ -373,10 +374,12 @@ def create(self, validated_data):
             config=validated_data["config"],
         )
 
-        # Attempt to assign a seat for this monitor
-        seat_outcome = quotas.backend.assign_seat(DataCategory.MONITOR, monitor)
-        if seat_outcome != Outcome.ACCEPTED:
-            monitor.update(status=ObjectStatus.DISABLED)
+        # Skip quota operations if requested by context (e.g., detector flow handles this)
+        if not self.context.get("skip_quota", False):
+            # Attempt to assign a seat for this monitor
+            seat_outcome = quotas.backend.assign_seat(DataCategory.MONITOR, monitor)
+            if seat_outcome != Outcome.ACCEPTED:
+                monitor.update(status=ObjectStatus.DISABLED)
 
         request = self.context["request"]
         signal_monitor_created(project, request.user, False, monitor, request)
@@ -636,9 +639,12 @@ def validate(self, attrs: dict[str, Any]) -> dict[str, Any]:
         if self.instance:
             monitor_instance = self.instance
 
+        # Skip quota operations - the detector validator handles seat assignment
+        context = {**self.context, "skip_quota": True}
+
         monitor_validator = MonitorValidator(
             data=monitor_data,
-            context=self.context,
+            context=context,
             instance=monitor_instance,
             partial=self.partial,
         )
@@ -686,16 +692,60 @@ class MonitorIncidentDetectorValidator(BaseDetectorTypeValidator):
 
     data_sources = serializers.ListField(child=MonitorDataSourceValidator(), required=False)
 
+    def validate_enabled(self, value: bool) -> bool:
+        """
+        Validate that enabling a detector is allowed based on seat availability.
+        """
+        detector = self.instance
+        if detector and value and not detector.enabled:
+            monitor = get_cron_monitor(detector)
+            result = quotas.backend.check_assign_seat(DataCategory.MONITOR, monitor)
+            if not result.assignable:
+                raise serializers.ValidationError(result.reason)
+        return value
+
+    def create(self, validated_data):
+        detector = super().create(validated_data)
+
+        with in_test_hide_transaction_boundary():
+            monitor = get_cron_monitor(detector)
+
+        # Try to assign a seat for the monitor
+        seat_outcome = quotas.backend.assign_seat(DataCategory.MONITOR, monitor)
+        if seat_outcome != Outcome.ACCEPTED:
+            detector.update(enabled=False)
+            monitor.update(status=ObjectStatus.DISABLED)
+
+        return detector
+
     def update(self, instance: Detector, validated_data: dict[str, Any]) -> Detector:
+        was_enabled = instance.enabled
+        enabled = validated_data.get("enabled", was_enabled)
+
+        # Handle enable/disable seat operations
+        if was_enabled != enabled:
+            monitor = get_cron_monitor(instance)
+
+            if enabled:
+                seat_outcome = quotas.backend.assign_seat(DataCategory.MONITOR, monitor)
+                # We should have already validated that a seat was available in
+                # validate_enabled, avoid races by failing here if we can't
+                # accept the seat
+                if seat_outcome != Outcome.ACCEPTED:
+                    raise serializers.ValidationError("Failed to update monitor")
+                monitor.update(status=ObjectStatus.ACTIVE)
+            else:
+                quotas.backend.disable_seat(DataCategory.MONITOR, monitor)
+                monitor.update(status=ObjectStatus.DISABLED)
+
         super().update(instance, validated_data)
 
         data_source_data = None
         if "data_sources" in validated_data:
             data_source_data = validated_data.pop("data_sources")[0]
 
         if data_source_data is not None:
-            data_source = DataSource.objects.get(detectors=instance)
-            monitor = Monitor.objects.get(id=data_source.source_id)
+            monitor = get_cron_monitor(instance)
 
             monitor_validator = MonitorDataSourceValidator(
                 instance=monitor,
@@ -709,3 +759,12 @@ def update(self, instance: Detector, validated_data: dict[str, Any]) -> Detector
                     monitor_validator.save()
 
         return instance
+
+    def delete(self) -> None:
+        assert self.instance is not None
+        monitor = get_cron_monitor(self.instance)
+
+        # Remove the seat immediately
+        quotas.backend.remove_seat(DataCategory.MONITOR, monitor)
+
+        super().delete()