Don't leak overwrite kwarg into plugin validated_data

The overwrite field on NoArtifactContentSerializer was declared with
default=True, which causes DRF to inject overwrite=True into
validated_data even when the caller omits it. This broke plugins that
splat **validated_data into a content model constructor (e.g. pulp_deb
sync), producing:

    TypeError: Package() got unexpected keyword arguments: 'overwrite'

Drop the field-level default and rely on the existing
validated_data.pop('overwrite', True) in create() to preserve the
'absent => True' semantics. No behavior change for API callers.

AI-Generated: github-copilot

Author: daviddavis

CHANGES/plugin_api/+overwrite-default-leak.bugfix

@@ -0,0 +1,3 @@
+Removed the `default=True` from the `overwrite` field on `NoArtifactContentSerializer` so it no
+longer leaks into `validated_data`. This fixes plugins that splat content serializer data
+(`**serializer.validated_data`).

pulpcore/app/serializers/content.py

@@ -35,7 +35,6 @@ class NoArtifactContentSerializer(base.ModelSerializer):
     )
     overwrite = serializers.BooleanField(
         required=False,
-        default=True,
         write_only=True,
         help_text=_(
             "When set to true, existing content in the repository with the same unique key "