Add cached app/config accessors in bionetgen.main

[wshlavacek]

Summary

• Add cached app and config accessors in bionetgen.main.
• Rename the exported accessor to get_default_app and add focused tests for caching and default bngpath lookup.
• Keep this PR limited to the accessor lane and leave deferred app setup changes for later work.

Testing

• uv run python -m pytest -q tests/test_main_accessors.py tests/test_bionetgen.py -k 'bionetgen_help or test_main_accessors'
• uvx black --check bionetgen/main.py tests/test_main_accessors.py