fix(tui): restore WSL clipboard image paste

[ForeverMJ]

Related Issue

Resolve #2722

Problem

Kimi Code running under WSL could not paste screenshots from the Windows
clipboard because the temporary image path was not forwarded across the
WSL interop boundary.

根因现已通过端到端单变量切换确认：

• 默认环境：稳定失败
• 仅将 KIMI_WSL_CLIPBOARD_IMAGE_PATH 加入 WSLENV：稳定成功
  因此 Clipboard image paste does not work in WSL #2722 的根因是：Kimi 把临时 PNG 路径放进普通 Linux 环境变量，但 WSL 启动 powershell.exe 时不会自动把该变量传给 Win32 进程。PowerShell 得不到 $path，保存失败；错误随后被转换成 null，TUI 再静默退回普通文本粘贴。

What changed

• Forward the PowerShell temporary image path through WSLENV with path conversion.
• Preserve and deduplicate existing WSLENV entries.
• Fall back to PowerShell when WSLg exposes only unsupported BMP clipboard data.
• Add regression coverage for WSL environment propagation and BMP-only WSLg clipboards.

Testing

• Related Vitest suites: 294 tests passed.
• TypeScript typecheck passed.
• Type-aware oxlint passed with no warnings or errors.
• Verified the real WSL PowerShell path returned a PNG from the Windows clipboard.

Checklist

• [√] I have read the CONTRIBUTING document.
• [√] I have linked a related issue, or explained the problem above.
• [√] I have added tests that prove my feature works.
• [√] Ran gen-changesets skill, or this PR needs no changeset.
• [√] Ran gen-docs skill, or this PR needs no doc update.