feat(models): improve EnrollmentFlow validation

when the flow's agency.active=True, validate that templates exist

benefits/core/models.py

@@ -592,13 +592,13 @@ def enrollment_success_template(self):
             return self.enrollment_success_template_override or f"{prefix}--{self.agency_card_name}.html"
 
     def clean(self):
-        supports_expiration = self.supports_expiration
-        expiration_days = self.expiration_days
-        expiration_reenrollment_days = self.expiration_reenrollment_days
-        reenrollment_error_template = self.reenrollment_error_template
+        errors = {}
+
+        if self.supports_expiration:
+            expiration_days = self.expiration_days
+            expiration_reenrollment_days = self.expiration_reenrollment_days
+            reenrollment_error_template = self.reenrollment_error_template
 
-        if supports_expiration:
-            errors = {}
             message = "When support_expiration is True, this value must be greater than 0."
             if expiration_days is None or expiration_days <= 0:
                 errors.update(expiration_days=ValidationError(message))
@@ -607,8 +607,25 @@ def clean(self):
             if reenrollment_error_template is None:
                 errors.update(reenrollment_error_template=ValidationError("Required when supports expiration is True."))
 
-            if errors:
-                raise ValidationError(errors)
+        if self.transit_agency and self.transit_agency.active:
+            self.transit_agency.clean()
+
+            templates = dict(
+                selection_label_template=self.selection_label_template,
+                eligibility_start_template=self.eligibility_start_template,
+                eligibility_unverified_template=self.eligibility_unverified_template,
+                enrollment_index_template=self.enrollment_index_template,
+                enrollment_success_template=self.enrollment_success_template,
+            )
+            if self.supports_expiration:
+                templates.update(reenrollment_error_template=self.reenrollment_error_template)
+
+            for name, tmpl in templates.items():
+                if not template_path(tmpl):
+                    errors.update({name: ValidationError(f"Template not found: {tmpl}")})
+
+        if errors:
+            raise ValidationError(errors)
 
     def eligibility_form_instance(self, *args, **kwargs):
         """Return an instance of this flow's EligibilityForm, or None."""