refactor: have the model property look up the context non-defensively

consumer is responsible for handling any error that might occur from
the key not being found.

benefits/core/models/enrollment.py

@@ -260,9 +260,7 @@ def enrollment_success_template(self):
 
     @property
     def in_person_eligibility_context(self):
-        system_name = self.system_name
-        eligibility_index = in_person_context.eligibility_index
-        return eligibility_index[system_name].dict() if system_name in eligibility_index.keys() else {}
+        return in_person_context.eligibility_index[self.system_name].dict()
 
     def clean(self):
         template_errors = []

benefits/in_person/forms.py

@@ -48,7 +48,10 @@ def __init__(self, agency: models.TransitAgency, *args, **kwargs):
         self.use_custom_validity = True
 
     def get_policy_details(self, flow: models.EnrollmentFlow):
-        eligibility_context = flow.in_person_eligibility_context
+        try:
+            eligibility_context = flow.in_person_eligibility_context
+        except KeyError:
+            eligibility_context = None
 
         return eligibility_context["policy_details"] if eligibility_context else None