Sandboxed Git credential flow fails with Avalonia RenderTimer error on macOS

## Summary

A Git operation that requires credentials fails when its interactive credential flow runs in the sandbox. Git Credential Manager attempts to start its Avalonia-based macOS UI, but Avalonia cannot start the native render timer and aborts. Subsequent credential and askpass fallbacks also fail, leaving Git without valid credentials.

## What happened

The credential request produced the following sequence:

- Avalonia threw `System.InvalidOperationException: Avalonia.Native was not able to start the RenderTimer. Native error code is: -6661` while initializing the Git Credential Manager UI.
- `git-credential-manager get` terminated with signal 6.
- A fallback attempted to create a temporary file under macOS's `/var/folders/.../T` directory and failed with `Operation not permitted`.
- An Electron credential/askpass path reported a macOS `SecCodeCheckValidity` error.
- The operation ended with `Missing or invalid credentials` and `Missing pipe`.

The `-6661` value corresponds to CoreVideo's invalid-argument error and occurs when Avalonia cannot create a display render timer, such as when no usable active display is available in a headless, sleeping-display, remote, VM, or restricted sandbox context. Although the sandbox command sets `TMPDIR`, the failing macOS-side helper uses its own process environment and macOS temporary directory.