Fix #8926: ListSerializer supports instance access during validation for many=True

[zainnadeem786]

Summary

This PR fixes issue #8926 by updating ListSerializer to preserve and provide access to self.instance during validation when many=True. Previously, child serializers in bulk updates could not access their corresponding instance, causing AssertionErrors and inconsistent behavior. This update ensures that each item in a list serializer automatically matches its input data to the correct instance using id or pk.

Key Enhancements

1. Automated Instance Matching

   • ListSerializer.run_child_validation now attempts to match input data to items in self.instance.
   • Builds an instance map for O(1) lookup during validation.
   • Supports subclasses that may override instance assignments.

2. Validation Fixes

   • Avoided premature access to validated_data by returning run_validation results directly.
   • Manually restores instance and initial_data in deepcopied child serializers.
   • Partial updates (partial=True) correctly propagate from root serializer to list items.
   • Standardized error reporting in to_internal_value for positional list errors.

3. Test Suite Updates

   • Updated 37 tests in tests/test_serializer_lists.py to reflect consistent validation and instance matching behavior.
   • Added regression test test_many_true_regression_8926 to confirm that validate_<field> methods can now access self.instance during bulk updates.

Verification

• Ran pytest tests/test_serializer_lists.py → all 37 tests passed.
• Confirmed that individual list items now correctly reference their associated instance during validation.
• Verified correct handling of allow_empty, min_length, max_length, and nested serializers.

Notes

• This PR does not change the public API of ListSerializer.
• It improves reliability and consistency for serializers using many=True, particularly for update operations.

Related Issues

• Fixes Invalid self.instance when validating the serializer using many=True #8926