Split out from #4741 to keep the main PR focused on installer logic.
- New test_install_llama_prebuilt_logic.py: tests for resolve logic,
fallback behavior, env_int, busy/lock handling
- New test_validate_llama_prebuilt.py: validator tests for staged
release_tag/upstream_tag handling
- New test_llama_pr_force_and_source.py: tests for PR_FORCE and
LLAMA_SOURCE maintainer defaults
- Updated test_selection_logic.py: expanded selection/fallback coverage
- Updated test_pr4562_bugfixes.py: updated bugfix tests for new logic
- Updated smoke_test_llama_prebuilt.py: minor update
f84c2d03d3