fixes #5542 fix(nimbus): Allow .0001 population_percent to be valid in ReadyForReview serializer (#5945)

Because:
* 0.0001 is a valid value that users can save, but the "Ready for review" validation errors show this to be an error

This commit:
* Adds a custom population_percent validation method to check for a min value rather than rely on min_value

app/experimenter/experiments/api/v5/serializers.py

@@ -560,7 +560,12 @@ class NimbusReadyForReviewSerializer(serializers.ModelSerializer):
     proposed_duration = serializers.IntegerField(required=True, min_value=1)
     proposed_enrollment = serializers.IntegerField(required=True, min_value=1)
     population_percent = serializers.DecimalField(
-        7, 4, min_value=0.0001, max_value=100.0, required=True
+        7,
+        4,
+        min_value=0.00009,
+        max_value=100.0,
+        required=True,
+        error_messages={"min_value": NimbusConstants.ERROR_POPULATION_PERCENT_MIN},
     )
     total_enrolled_clients = serializers.IntegerField(required=True, min_value=1)
     firefox_min_version = serializers.ChoiceField(

app/experimenter/experiments/constants/nimbus.py

@@ -418,6 +418,7 @@ Optional - We believe this outcome will <describe impact> on <core metric>
     ERROR_REQUIRED_FEATURE_CONFIG = (
         "You must select a feature configuration from the drop down."
     )
+    ERROR_POPULATION_PERCENT_MIN = "Ensure this value is greater than or equal to 0.0001."
 
     # Analysis can be computed starting the week after enrollment
     # completion for "week 1" of the experiment. However, an extra

app/experimenter/experiments/tests/api/v5/test_serializers.py

@@ -1608,6 +1608,21 @@ class TestNimbusReadyForReviewSerializer(TestCase):
             "Ensure this value is greater than or equal to 0.0001.",
         )
 
+    def test_valid_experiment_minimum_population_percent(self):
+        experiment = NimbusExperimentFactory.create_with_lifecycle(
+            NimbusExperimentFactory.Lifecycles.CREATED,
+            population_percent=0.0001,
+        )
+        serializer = NimbusReadyForReviewSerializer(
+            experiment,
+            data=NimbusReadyForReviewSerializer(
+                experiment,
+                context={"user": self.user},
+            ).data,
+            context={"user": self.user},
+        )
+        self.assertTrue(serializer.is_valid())
+
     def test_invalid_experiment_treatment_branch_requires_description(self):
         experiment = NimbusExperimentFactory.create_with_lifecycle(
             NimbusExperimentFactory.Lifecycles.CREATED,