Remote settings clients now stop refreshing after the process exits

What

• The remote settings/eval class now tracks processExitListenerInstalled and processExited flags, along with an onProcessExit handler that marks itself as exited and stops its periodic refresh. A related class also gained a backstopReleased flag, set when a load-promise safety timeout expires and resolves anyway.
• The GrowthBook client's createClient() now installs a one-time process exit listener, and its startPeriodicRefresh() now checks the new processExited flag before starting any refresh loop.

Why

This stops these clients from trying to do background refresh work after the process has already exited, avoiding wasted work or errors during shutdown.