Add unit test for get_issues_by_ids 'raise on unresolved' behavior

[gaurav]

Background

GitHubIssuesTestCases.get_issues_by_ids() raises a ValueError when an issue ID string cannot be resolved to a GitHub issue in any configured repository. This was added in PR #67 but is not covered by the existing unit tests in tests/test_environment/test_system.py.

What's missing

A unit test (mock the PyGitHub get_repo / get_issue calls) that:

1. Passes an unresolvable issue ID (e.g. "nonexistent-repo#999" or "99999" with no matching repo).
2. Asserts a ValueError is raised with a message mentioning the unresolvable ID.

A happy-path test for each of the three ID formats (org/repo#N, repo#N, N) would also be valuable.

Notes

• Identified during the Opus code review of PR Add GitHub issue tests #67.
• The logic lives in src/babel_validation/sources/github/github_issues_test_cases.py::get_issues_by_ids.