refactor: Define metadata field in MetadataSerializerMixin

Make MetadataSerializerMixin inherit from Serializer and define the
metadata field directly in the mixin. This eliminates the need to
manually define 'metadata = MetadataSerializer(required=False, many=True)'
in every serializer that uses the mixin.

Changes:
- MetadataSerializerMixin now inherits from serializers.Serializer
- Added metadata field definition to the mixin
- Removed redundant metadata field declarations from:
  - SegmentSerializer
  - FeatureSerializerWithMetadata
  - EnvironmentSerializerWithMetadata
- Removed unnecessary MetadataSerializer imports

Co-authored-by: Claude <[email protected]>

Author: emyller

api/environments/serializers.py

@@ -4,7 +4,7 @@
 
 from environments.models import Environment, EnvironmentAPIKey, Webhook
 from features.serializers import FeatureStateSerializerFull
-from metadata.serializers import MetadataSerializer, MetadataSerializerMixin
+from metadata.serializers import MetadataSerializerMixin
 from organisations.models import Subscription
 from organisations.subscriptions.serializers.mixins import (
     ReadOnlyIfNotValidPlanMixin,
@@ -79,8 +79,6 @@ class EnvironmentSerializerWithMetadata(
     DeleteBeforeUpdateWritableNestedModelSerializer,
     EnvironmentSerializerLight,
 ):
-    metadata = MetadataSerializer(required=False, many=True)
-
     class Meta(EnvironmentSerializerLight.Meta):
         fields = EnvironmentSerializerLight.Meta.fields + ("metadata",)  # type: ignore[assignment]
 

api/features/serializers.py

@@ -23,7 +23,7 @@
 )
 from integrations.github.constants import GitHubEventType
 from integrations.github.github import call_github_task
-from metadata.serializers import MetadataSerializer, MetadataSerializerMixin
+from metadata.serializers import MetadataSerializerMixin
 from projects.code_references.serializers import (
     FeatureFlagCodeReferencesRepositoryCountSerializer,
 )
@@ -345,8 +345,6 @@ def get_last_modified_in_current_environment(
 
 
 class FeatureSerializerWithMetadata(MetadataSerializerMixin, CreateFeatureSerializer):
-    metadata = MetadataSerializer(required=False, many=True)
-
     code_references_counts = FeatureFlagCodeReferencesRepositoryCountSerializer(
         many=True,
         read_only=True,

api/metadata/serializers.py

@@ -103,11 +103,16 @@ def validate(self, attrs: dict[str, Any]) -> dict[str, Any]:
         return attrs
 
 
-class MetadataSerializerMixin:
+class MetadataSerializerMixin(serializers.Serializer):  # type: ignore[type-arg]
     """
-    Functionality for serializers that need to handle metadata
+    Mixin for serializers that need to handle metadata.
+
+    Automatically adds a 'metadata' field to any serializer that inherits from this mixin.
+    Child serializers should include 'metadata' in their Meta.fields.
     """
 
+    metadata = MetadataSerializer(required=False, many=True)
+
     def _validate_required_metadata(
         self, organisation: Organisation, metadata: list[dict[str, Any]]
     ) -> None:

api/segments/serializers.py

@@ -7,7 +7,7 @@
 from rest_framework import serializers
 from rest_framework.exceptions import ValidationError
 
-from metadata.serializers import MetadataSerializer, MetadataSerializerMixin
+from metadata.serializers import MetadataSerializerMixin
 from projects.models import Project
 from segments.models import Condition, Segment, SegmentRule
 
@@ -80,7 +80,6 @@ class Meta:
 
 class SegmentSerializer(MetadataSerializerMixin, WritableNestedModelSerializer):
     rules = SegmentRuleSerializer(many=True, required=True, allow_empty=False)
-    metadata = MetadataSerializer(required=False, many=True)
 
     def __init__(self, *args: Any, **kwargs: Any) -> None:
         """