Preserve Windows proxy settings in Guardian sessions (#40641)

## Why

Guardian sessions can run alongside another Windows sandbox launch and should not reconcile the persistent proxy settings established by that launch.

## What changed

- Start internal Guardian sessions with `WindowsSandboxProxySettingsMode::Preserve`.
- Continue using `Reconcile` for other sessions.

## Testing

Added a thread manager test that verifies a parent session reconciles proxy settings while its internal Guardian session preserves them.

GitOrigin-RevId: c6a31637ecdf828548cfd7153da7e9405abe34fa