fix: fall back to python pagerank when scipy is missing

[MobiusL00p]

Summary

• catch ModuleNotFoundError from networkx.pagerank() when the scipy-backed implementation is unavailable
• fall back to NetworkX's pure-Python PageRank implementation so repo map generation still works
• add a regression test that simulates missing scipy and verifies the fallback path is used

Verification

• python -m pytest tests/basic/test_repomap.py -k scipy
• python -m pytest tests/basic/test_repomap.py -k "scipy or get_repo_map_excludes_added_files or get_repo_map_with_identifiers"
• python -m compileall aider/repomap.py tests/basic/test_repomap.py

Closes #4962.