fix: retry commit messages after blank responses

[primorLee]

Summary

• normalize each generated commit message before deciding whether fallback is needed
• continue to the next configured model when a response contains only whitespace
• add regression coverage for spaces, tabs, and newlines

Fixes #5581

Validation

• python -m pytest tests/basic/test_repo.py::TestRepo::test_get_commit_message_skips_whitespace_only_response -q — 1 passed
• python -m pytest tests/basic/test_repo.py -q — 17 passed, 5 skipped on Windows
• python -m flake8 aider/repo.py tests/basic/test_repo.py --show-source
• python -m black --check --line-length 100 --preview aider/repo.py tests/basic/test_repo.py
• python -m isort --check-only --profile black aider/repo.py tests/basic/test_repo.py

Implementation and regression tests were prepared with AI assistance and manually validated against the reported fallback behavior.