The gRPC streaming code was using tokio::runtime::Handle::current().block_on()
inside filter_map closures, which caused a panic ('Cannot start a runtime from
within a runtime') when called from an async context.
Fixed by replacing the pattern with .then(async move { ... }).filter_map(|x| x)
which properly handles async operations in stream pipelines.
This fixes the gRPC Ping/Pong freeze issue and restores request cancellation.
