fix(macos): stop writing LLM_BACKEND twice into generated env

[patil2001]

Summary

Windows PowerShell 5.1 decodes native command output as ANSI while wsl.exe -l -q emits UTF-16LE, so detected distro names arrived NUL-padded ("U\0b\0u\0n\0t\0u"). The padded string was handed straight to wsl.exe -d, failing distro selection. Names are now stripped of NUL padding before use.

AI Assistance

AI-assisted confirmation of wsl.exe output encoding behavior. The author reviewed the decode path and is responsible for the final diff.

Release Lane

• Stable hotfix targeting release/2.6.x
• Mainline change targeting main
• Next-minor work targeting the next feature/minor release
• Not sure; reviewer should help classify

Stable hotfix reason:

Not targeting the stable release branch directly.

Changed Surface

• Docs only
• Tests only
• Dashboard UI
• Installer
• Core runtime
• Dashboard API
• CI workflows

Validation

• Diff reviewed against PS 5.1 native-output decoding semantics; PSScriptAnalyzer (lint-powershell.yml) will parse-check in CI (no local pwsh).