Lazy-load session backends and monitors

[Zhipeng Wang (timenick)]

Summary

• Lazy-load the session, monitor, and sysinfo public facades while preserving their existing exports.
• Defer QAIRT and provider-specific monitor modules until the matching backend is selected.
• Add subprocess import-isolation coverage for Runtime, GenAI, QNN, OpenVINO, VitisAI, and QAIRT, including actionable missing-dependency diagnostics.

Closes #1381

Validation

• 15 import-isolation tests passed.
• CI-equivalent commands/session shard: 3667 passed.
• CI-equivalent remaining shard: 960 passed.
• Mypy checked 441 source files.
• Pre-commit passed.