Plugin manager: use info instead of error for plugins missing external deps (#964)

[ovidiusm]

Cherry pick of #964

What?

Python wheel does not bundle libfabric dependencies. As a result, we get log errors on non-AWS systems:

+ python /workspace/examples/python/nixl_api_example.py
2025-10-28 10:54:56 NIXL INFO    nixl_api_example.py:35 Using NIXL Plugins from:
default location
E1028 10:54:56.298455       1 nixl_plugin_manager.cpp:122] Failed to load plugin from /workspace/venv/lib/python3.12/site-packages/nixl_cu12/../.nixl_cu12.mesonpy.libs/plugins/libplugin_LIBFABRIC.so: /opt/amazon/efa/lib/libfabric.so.1: version `FABRIC_1.8' not found (required by /workspace/venv/lib/python3.12/site-packages/nixl_cu12/../.nixl_cu12.mesonpy.libs/plugins/libplugin_LIBFABRIC.so)
E1028 10:54:56.298478       1 nixl_plugin_manager.cpp:288] Failed to load plugin 'LIBFABRIC' from any directory

These logs should be INFO.

[github-actions]

👋 Hi ovidiusm! Thank you for contributing to ai-dynamo/nixl.

Your PR reviewers will review your contribution then trigger the CI to test your changes.

🚀