Fix io_runtime drop panic in test_postgres_io_runtime_segregation

[alexanderbianchi]

Summary

Fixes the test_postgres_io_runtime_segregation test panic mentioned in #559.

The test creates a tokio::runtime::Runtime (io_runtime) inside an async fn. When the function returns, Rust drops io_runtime, and Runtime::drop performs a blocking shutdown to wait for spawned tasks. Since the test runs on a Tokio worker thread (via #[tokio::test]), blocking is not allowed — causing the panic:

Cannot drop a runtime in a context where blocking is not allowed.
This happens when a runtime is dropped from within an asynchronous context.

The fix calls io_runtime.shutdown_background() before the end of the test, which consumes the runtime and initiates a non-blocking shutdown, avoiding the panic.

Test plan

• test_postgres_io_runtime_segregation no longer panics