Drop global Rust reader runtime

[sebastian-flyr]

Summary

• Replace the process-global Tokio runtime with a runtime owned by each Rust PyReader.
• Let Rust reader runtime/thread state be dropped with the reader instead of persisting for the Python process lifetime.
• Avoid leaving native runtime threads around after an engine=\"rust\" read, which makes mixed engine usage safer for callers that later use the Python multiprocessing path.

Test plan

• cargo check --manifest-path /tmp/pyarrow-bigquery/rust/Cargo.toml
• Installed local package into the consuming pre-dqn project and ran same-process mixed engine benchmarks.
• Mixed benchmark no longer shows the previous Python-after-Rust degradation from ~400-500s; Python-after-Rust now matches Python-only repeated-read behavior around ~102-105s.
• Rust-only repeated reads stayed stable around ~24-29s.