Extensions: Check for Extension Updates does not update private git-subdir agent plugins

# `Chat: Update Plugins` does not update private agent plugins

## Summary

VS Code does not update a private agent plugin sourced from a Git subdirectory, even when the plugin's `plugin.json` version has increased and a newer commit is available on the configured branch.

## Environment

- VS Code: `1.131.0`
- OS: macOS
- Plugin feature: Agent Plugins (Preview)

## Marketplace Configuration

`marketplace.json` plugin entry:

```json
{
"name": "example-agent-plugin",
"source": {
"source": "git-subdir",
"url": "https://github.com/example-organization/example-agent-plugins.git",
"path": "plugins/example-agent-plugin",
"ref": "main"
}
}
```

The plugin repository is private, but Git authentication is configured and works normally from the terminal.

## Steps To Reproduce

1. Install an agent plugin from a private marketplace using a `git-subdir` source.
2. Push a change to the plugin's tracked branch.
3. Bump the plugin `version` in its root `plugin.json`, for example from `1.1.8` to `1.1.9`.
4. In VS Code, run `Extensions: Check for Extension Updates`.
5. Check the installed plugin version and cache.

## Expected Behavior

VS Code detects the new `plugin.json` version, fetches the configured branch, and updates the installed plugin.

## Actual Behavior

No update is found or applied.

The installed plugin checkout remained behind the remote branch:

```text
## main...origin/main [behind 2]
```

The installed manifest remained at `1.1.8`, while `origin/main` had `1.1.9`.

A manual pull of the exact plugin cache succeeds immediately:

```bash
git -C ~/.vscode/agent-plugins/github.com/example-organization/example-agent-plugins/ref_main pull --ff-only origin main
```

After that command, the installed manifest reports:

```text
installed plugin version: 1.1.9
```

## Additional Context

- The source repository can be cloned and pulled via HTTPS.
- The local plugin checkout has the expected remote: `https://github.com/example-organization/example-agent-plugins.git`.
- The plugin is installed from the `main` branch.

This appears to be a failure in VS Code's agent-plugin update check for a private Git-backed `git-subdir` source.