test: cover PR/MR platform helpers

[sudorest]

Summary

• add regression coverage for PR/MR URL parsing, labels, display helpers, and CLI selection across GitHub and GitLab
• include self-hosted GitLab parsing coverage to protect the platform routing introduced in feat: GitLab merge request review support #364
• migrate the review editor's open-PR preference to a platform-neutral storage key while preserving reads from the legacy GitHub-specific key

Verification

• bun test packages/shared/pr-provider.test.ts packages/shared/review-core.test.ts

Context

Follow-up to #364 to protect the new GitHub/GitLab review abstraction with focused helper coverage and a small platform-neutral naming cleanup.