Fix ViewInspector sharing view state across views (#6877)

[abdurrahimcs50]

Problem

Fixes #6877.

A single ViewInspector instance (e.g. an AutoSchema assigned via @action(schema=AutoSchema())) can end up attached to multiple views when the action is defined on a mixin that's reused across several ViewSets, since the decorator only evaluates AutoSchema() once, at decoration time.

ViewInspector.__set__ stored view as a plain mutable attribute directly on that shared instance, so each view that touched it during schema generation would overwrite .view for every other view sharing the same instance. The result: schema generation collapses onto whichever view happened to set it last, producing duplicate/incorrect operationIds, descriptions, and request/response schemas for the other views.

Reproduction

class CancelViewSetMixin:
    @action(methods=['post'], detail=True, schema=AutoSchema())
    def cancel(self, request, pk):
        return Response()

class ViewSetA(CancelViewSetMixin, GenericViewSet):
    serializer_class = ASerializer

class ViewSetB(CancelViewSetMixin, GenericViewSet):
    serializer_class = BSerializer

Before this fix, generating the schema for both viewsets produces the same operationId for both cancel actions (and DRF itself warns about the resulting duplicate operationId).

Fix

__set__ now makes a shallow copy.copy() of the schema instance before binding .view to it, so each view gets its own independent copy instead of mutating shared state.

Testing

• Added tests/schemas/test_view_inspector.py, reproducing the exact mixin scenario from Custom action in a mixin class with custom schema results in incorrect schema generation #6877 and asserting each view gets a distinct operationId.
• Full test suite passes (./runtests.py).
• pre-commit passes.