[FEA]: Add CI check for unqualified uses of `cuda::` namespace

### Is this a duplicate?

- [X] I confirmed there appear to be no [duplicate issues](https://github.com/NVIDIA/cccl/issues) for this request and that I agree to the [Code of Conduct](CODE_OF_CONDUCT.md)

### Area

General CCCL

### Is your feature request related to a problem? Please describe.

Inspired by https://github.com/NVIDIA/cccl/pull/1705, we should always fully qualify uses of the `cuda::` namespace as `::cuda::`.

### Describe the solution you'd like

Add a CI check that greps for unqualified uses of `cuda::` namespace.

This check should ignore tests and samples.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_