fix(nbd): change NBD status poll sleep from 100ns to 100µs to avoid useless busy spinning

[AdaAibaby]

fix(nbd): adjust status poll sleep from 100ns to 100µs

Optimize NBD connected check polling interval:

• Original 100ns causes useless busy spin since nbdnl.Connect() is sync netlink call;
• New 100µs yields CPU safely without any functional regression;
• Open/Close path have ctx & 30s timeout guard, rootfs start runs in separate goroutine, no side effect for sandbox rootfs lifecycle.
  /cc @jakubno

[gemini-code-assist]

Code Review

This pull request updates the polling sleep interval in path_direct.go from 100 nanoseconds to 100 microseconds during NBD connection and disconnection retries, likely to reduce CPU usage. There are no review comments, and I have no feedback to provide.