test(desktop): cover stable package index

[tulayha]

Summary

• make the desktop package-index contract test channel-aware
• keep TestPyPI for prerelease packages and PyPI for stable packages
• verify package acquisition uses the index selected for the current manifest

Validation

• cargo +1.97.1 test --release --locked --manifest-path desktop/src-tauri/Cargo.toml tests::package_and_dependencies_use_channel_specific_indexes -- --exact
• repeated with a temporary stable 0.3.0 package_version in desktop/runtime-manifest.json, then restored the manifest