testing: cross-platform integration test coverage

[SebTardif]

Summary

Unit tests now run on Ubuntu, macOS, and Windows via the CI matrix (issue #25). However, VS Code extension integration tests and UI tests only run on the self-hosted macOS ARM64 runner.

Gap

Platform-specific differences in VS Code API behavior (file paths, URI handling, shell execution) are not tested on Linux or Windows. The extension supports remote environments (WSL, SSH, dev containers) which are primarily Linux-based.

Considerations

• Integration tests require @vscode/test-electron which needs a display (Xvfb on Linux, screen on Windows)
• UI tests use ExTester and depend on VS Code's native window management
• The self-hosted runner handles macOS-specific window suppression via scripts/hide-test-vscode.sh

Proposed approach

1. Add a Linux integration test job using ubuntu-latest with xvfb-run
2. Consider Windows integration tests if significant user adoption occurs on that platform
3. Keep UI tests (ExTester) on macOS self-hosted only since they depend on native window management

Priority

Low. Unit tests catch most logic issues. Integration tests are primarily validating VS Code API interactions which are largely platform-independent.

[SebTardif]

Cross-platform integration tests depend on GitHub-hosted runners (for xvfb-run on ubuntu-latest). These runners cannot start for this private repo due to billing ('recent account payments have failed or your spending limit needs to be increased'). Same blocker as #25. Unblocks when billing is resolved or the repo goes public.

[SebTardif]

Status update (current session):

Still blocked on the same GitHub billing constraint as #25.

• Unit tests now have a documented path (reverted in 895a8d4) but integration/UI tests require the self-hosted macOS runner for display/Xvfb and native window behavior.
• Proposed Linux job with xvfb-run cannot run because ubuntu-latest cannot be allocated.
• UI tests (ExTester) are macOS-specific in practice due to window suppression script.

Unblocks with public repo or billing resolution. Low priority relative to core features once unblocked.

Related to #25 and #43.