ref(workflow_engine): Remove DetectorType (#82111)

## Description
Removes the `DetectorType` from the `workflow_engine.type` and adds
comments to clarify how this type is defined and used.

src/sentry/workflow_engine/models/detector.py

@@ -55,8 +55,7 @@ class Detector(DefaultFieldsModel, OwnerModel, JSONConfigBase):
         on_delete=models.SET_NULL,
     )
 
-    # The type of detector that is being used, this is used to determine the class
-    # to load for the detector
+    # maps to registry (sentry.issues.grouptype.registry) entries for GroupType.slug in sentry.issues.grouptype.GroupType
     type = models.CharField(max_length=200)
 
     # The user that created the detector
@@ -106,8 +105,14 @@ def get_audit_log_data(self) -> dict[str, Any]:
 
 @receiver(pre_save, sender=Detector)
 def enforce_config_schema(sender, instance: Detector, **kwargs):
+    """
+    Ensures the detector type is valid in the grouptype registry.
+    This needs to be a signal because the grouptype registry's entries are not available at import time.
+    """
     group_type = instance.group_type
+
     if not group_type:
         raise ValueError(f"No group type found with type {instance.type}")
+
     config_schema = group_type.detector_config_schema
     instance.validate_config(config_schema)

src/sentry/workflow_engine/types.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from enum import IntEnum, StrEnum
+from enum import IntEnum
 from typing import TYPE_CHECKING, Any, Generic, TypeVar
 
 from sentry.types.group import PriorityLevel
@@ -44,8 +44,3 @@ class DataConditionHandler(Generic[T]):
     @staticmethod
     def evaluate_value(value: T, comparison: Any, condition: str) -> DataConditionResult:
         raise NotImplementedError
-
-
-class DetectorType(StrEnum):
-    ERROR = "ErrorDetector"
-    METRIC_ALERT_FIRE = "metric_alert_fire"