Cache Server Graceful Shutdown

Provider Cache Server now drains pending downloads on shutdown

When Terragrunt finished a run with the Provider Cache Server enabled, the cache server stopped instantly rather than letting in-flight provider downloads complete. Downloads still being served to OpenTofu/Terraform were cut off mid-response, and successful runs ended with a context canceled error in the logs.

Terragrunt now waits for those downloads to finish before stopping the cache server, and a graceful shutdown is no longer reported as an error.