Implement GPU environment verification in session management

- Added a new function `verify_gpu_env` to check GPU stack readiness, including nvidia-smi, CUDA, and torch in the Comfy virtual environment when SwarmUI is enabled.
- Updated the session management to call `verify_gpu_env`, capturing GPU environment status and errors in the state notes.
- Enhanced documentation in `cli.md` to reflect the new GPU environment verification process.
- Added tests for `verify_gpu_env` to ensure proper functionality and error handling during GPU checks.

tests/test_session.py

@@ -48,6 +48,18 @@ def _mock_bind(monkeypatch):
     )
     monkeypatch.setattr("gpu_rent.session.provision_vm", lambda cfg, host, log, **kw: None)
     monkeypatch.setattr("gpu_rent.session.wait_backend_idle", lambda cfg, host, log, **kw: None)
+    monkeypatch.setattr(
+        "gpu_rent.session.verify_stack_on_vm",
+        lambda cfg, host, log, **kw: [],
+    )
+    monkeypatch.setattr(
+        "gpu_rent.session.verify_gpu_env",
+        lambda cfg, host, log, **kw: [],
+    )
+    monkeypatch.setattr(
+        "gpu_rent.session.tune_swarm_perf",
+        lambda cfg, host, log: False,
+    )
     monkeypatch.setattr(
         "gpu_rent.session.ensure_boot_snapshot",
         lambda conn, boot_volume_id, cfg, log: None,