Fix Rust SDK availability probe

[BrettKinny]

Summary

• let mise-managed SDKs specify the executable used for observed-state validation
• validate Rust through rustc instead of the nonexistent rust executable
• cover successful Rust installation and Selection commit behavior

Root cause

Squarebox passed the mise tool name to mise which. That works where the tool and executable share a name, but Rust installs rustc and cargo, not a rust binary.

Validation

• all executable shell tests passed
• focused provisioning regression passed
• shell syntax checks passed
• PowerShell coverage remained platform-skipped because pwsh is unavailable