[v11] Sparse fieldsets allow for invalid types

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Description

When specifying sparse fieldsets, if you specify the type as the singular version of the type (i.e. fields[user]=name instead of fields[users]=name, the request succeeds instead of returning a 400.

Bug reports:

On the v0-11-dev branch with Ruby 3.3.5. The root of the issue is that classify is used to look up the resource class, which works the same for user and users, they both become UserResource, but then parse_fields doesn't verify that the type matches after getting the resource class back.

On the v0-11-dev branch, locate parse_fields and trace how the sparse fieldset type is resolved to a resource class. Add coverage for fields[user]=name and verify that the singular type returns HTTP 400 while the valid plural form continues to succeed.