Add CUDA build and GPU test run to CI/CD pipeline

[kaanolgu]

Description

Following the fprettify issue discovered in #272, we currently lack CI/CD coverage for the CUDA backend. This means build-breaking changes (like the triple-chevron syntax corruption) can go unnoticed until manual testing is performed.

Proposal

Add a CI/CD pipeline stage that:

• Builds the project with the NVHPC compiler targeting the CUDA backend
• Runs the test suite on a GPU system

Motivation

• Fprettify breaks cuda calls #272 would have been caught automatically before merging if a CUDA build step existed in CI
• Ensures future changes don't silently break GPU support