Fix viewset actions dict being mutated after first request

[mag123c]

Fix the original actions dict passed to ViewSet.as_view() being mutated after the first request.

refs #9747

[Copilot]

Pull request overview

This PR fixes a bug where the original actions dictionary passed to ViewSet.as_view() was being mutated after the first request, specifically by adding a 'head' key when a 'get' action was present.

• Creates a copy of the actions dictionary before any modifications
• Updates all internal references to use the copied dictionary
• Adds a regression test to ensure the original dictionary remains unchanged

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
rest_framework/viewsets.py	Creates a copy of the actions dict (bound_actions) before modification and uses it for all internal operations, preventing mutation of the original dict
tests/test_viewsets.py	Adds a regression test to verify the original actions dict is not mutated after calling as_view() or making requests

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