test(iceberg): reproduce worker FileIO runtime mismatch

[alexanderbianchi]

Summary

Adds an ignored reproducer for worker-side Iceberg FileIO being polled on a query runtime with Tokio IO disabled, even when IcebergCodec is configured with a separate IO-enabled iceberg::Runtime.

The test configures an S3-backed FileIO exactly where the codec reconstructs it. Running it explicitly panics while opening the TCP connection:

A Tokio 1.x context was found, but IO is disabled. Call `enable_io` on the runtime builder to enable IO.

Reproduce

cargo test -p datafusion-distributed-iceberg \
  codec::tests::decoded_file_io_uses_configured_io_runtime \
  -- --ignored --exact --nocapture

The test is ignored so this draft does not make the normal suite fail. It contains no fix.