Add direct repo-scope fallback coverage in Rust guard

[Copilot]

extract_repo_scope_with_query_fallback is security-relevant for issue and PR collection labeling, but only had indirect coverage through larger response-labeling tests. This change adds direct coverage for its arg-vs-query resolution behavior and simplifies duplicated items_path handling in response_paths.rs.

• Direct coverage for repo-scope fallback

  • Adds focused unit tests for the four resolution paths in extract_repo_scope_with_query_fallback:
    • explicit owner/repo wins over query
    • fallback to query when args are absent
    • fallback to query when args are partial
    • empty result when neither source provides repo scope

• Idiomatic items_path construction

  • Replaces three repeated if items_path.is_empty() { None } else { Some(items_path) } blocks with then_some(...)
  • Keeps behavior unchanged while removing duplicated branching in the PR, issue, and project-item response path code

• Example

  items_path: (!items_path.is_empty()).then_some(items_path),