test(ads-client): add integration tests against MARS staging with GitHub CI workflow

[Almaju]

Summary

• Refactors existing #[ignore] integration tests to use viaduct-hyper and MozAdsEnvironment::Staging for Image, Spoc, and Tile ad types
• Adds a GitHub Actions workflow (.github/workflows/ads-client-tests.yaml) that runs these tests on every PR/push touching components/ads-client/**
• Run manually with: cargo test -p ads-client --test integration_test -- --ignored

Split from #7263

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