Add a compatibility setting for ListSerializer error formats

[Kub-AT]

Description

Follow-up to #9837 and #10018.

This adds LIST_SERIALIZER_ERRORS_AS_DICT. It defaults to True, preserving the dictionary format introduced in 3.18.0.

Projects can set it to False to temporarily restore the old list-based format while migrating. This emits RemovedInDRF320Warning, and the list format will be removed in 3.20.

[Copilot]

Pull request overview

Adds a REST framework setting to control the ListSerializer (many=True) per-item validation error format, restoring the legacy list-based structure as the default while allowing opt-in to the newer dict-based format.

Changes:

• Introduces REST_FRAMEWORK['LIST_SERIALIZER_ERRORS_AS_DICT'] (default False) and updates ListSerializer.to_internal_value() to emit legacy list-form errors (with a deprecation warning) unless the dict format is enabled.
• Updates and expands serializer list-related tests to cover both formats and warning behavior.
• Documents the new setting and the deprecation/transition plan in the settings and serializers guides.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/test_serializer.py	Adjusts a regression test to expect the new deprecation warning alongside the raised validation error.
tests/test_serializer_lists.py	Adds coverage for default legacy list errors, opt-in dict errors, and explicit legacy setting behavior.
tests/test_serializer_bulk_update.py	Updates expected bulk many=True error shapes to match the restored legacy list format and asserts warnings.
rest_framework/settings.py	Adds the new LIST_SERIALIZER_ERRORS_AS_DICT default setting.
rest_framework/serializers.py	Implements conditional legacy-vs-dict error formatting and emits a deprecation warning for the legacy default.
rest_framework/init.py	Adds RemovedInDRF320Warning warning class used for the deprecation signal.
docs/api-guide/settings.md	Documents the new LIST_SERIALIZER_ERRORS_AS_DICT setting and its behavior.
docs/api-guide/serializers.md	Updates serializer error-format documentation to note deprecation and explain enabling dict-based errors.

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[browniebroke]

Should we do a 3.18.1 release for this? I think it blocks a few people from upgrading... You can start here @auvipy