[Bug] Extension install fails instantly (0ms) with no error_message on ComfyUI Desktop macOS — operation returns failed but nothing happens

## Bug Description

On ComfyUI Desktop (macOS, arm64), attempting to install any custom node via Manager fails immediately with no actionable error. The operation completes in 0ms, `result` is set to `"failed"`, but `error_message` is `null`.

## Environment

- Platform: macOS Darwin 25.4.0 (arm64)
- ComfyUI Desktop 0.8.36
- Python: 3.12.11 (via ComfyUI venv at `~/Documents/ComfyUI/.venv/bin/python`)
- Manager version: V4.2.1
- ComfyUI root: `/Applications/ComfyUI.app/Contents/Resources/ComfyUI`
- `git_exe` in config.ini: `/usr/bin/git` (explicitly set)
- `use_uv`: true

## Batch History Evidence

Install operations produce entries like this:

```json
{
"operation_id": "...",
"operation_type": "install",
"target": "task_...",
"result": "failed",
"error_message": null,
"start_time": "2026-05-05 13:32:27.652231",
"end_time": "2026-05-05 13:32:27.652231" // 0ms — instant failure
}
```

Note that `start_time == end_time`, meaning the operation didn't even begin executing.

## Manager Log

Manager log shows:
```
[ComfyUI-Manager] Using uv as Python module for pip operations.
** Python version: 3.12.11
** Python executable: /Users/roriksun/Documents/ComfyUI/.venv/bin/python
[ComfyUI-Manager] Skipped fixing the 'comfyui-frontend-package' dependency because the ComfyUI is outdated.
[ComfyUI-Manager] default cache updated: https://api.comfy.org/nodes
```

No error is logged when the install operation fails — the `error_message` field is simply `null`.

## Batch History `approval` Field

The batch history JSON also contains a suspicious `approval` field:
```json
"approval": "Command required approval (Security scan — [HIGH] Pipe to interpreter: cat | python3: Command pipes output from 'cat' directly to interpreter 'python3'. Downloaded content will be executed without inspection.) and was approved by the user."
```

This suggests the security scan is triggering on something, but the actual failure reason is swallowed and not surfaced to the user.

## Expected Behavior

User should see an actual error message explaining why the install failed (e.g., git not found, network error, permission denied, security gate, etc.).

## Actual Behavior

- Install button triggers a "please restart" message
- After restart, nothing is installed
- `error_message` is `null` in batch history — no way to diagnose what happened

## Steps to Reproduce

1. Launch ComfyUI Desktop on macOS
2. Open Manager → Nodes Manager
3. Click Install on any extension (e.g. ComfyUI-KJNodes)
4. Accept the restart prompt
5. After restart: extension is not installed, no error shown

## Workaround

Manual `git clone` into `custom_nodes/` works correctly. Manager's built-in install mechanism is completely non-functional.