Fix NestedBoundField child access after parent validation error

[Shrikantgiri25]

When a nested serializer raises a parent-level ValidationError, the resulting
error value is a list rather than a mapping. Accessing child bound fields in this
case causes NestedBoundField.__getitem__() to pass None for errors, breaking
the bound-field contract.

This change normalizes list-based errors to an empty dict when resolving child
bound fields, ensuring they remain accessible.

Adds a regression test for issue #4073.

[Copilot]

Pull request overview

This PR fixes a bug where accessing child bound fields fails after a parent-level validation error in nested serializers. When a nested serializer raises a parent-level ValidationError (resulting in a list rather than a dict), the NestedBoundField.__getitem__() method was passing None for errors, breaking the bound-field contract.

Key changes:

• Normalizes list-based errors to an empty dict when resolving child bound fields
• Adds regression test for issue #4073

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
rest_framework/utils/serializer_helpers.py	Updated NestedBoundField.__getitem__() to handle list-based errors by normalizing them to an empty dict
tests/test_bound_fields.py	Added regression test verifying child bound fields remain accessible after parent-level validation errors

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