fix: keep Vortex runtime alive during async IO

[QuakeWang]

Purpose

Fixes the Ubuntu unit test hang in Vortex format tests.

The previous code used VortexSession::default() directly. In Vortex 0.68, runtime handles are weak references to the underlying executor, so async read/write work can outlive the temporary runtime handle owner and stall when Vortex later spawns background IO tasks.

Brief change log

• Create Vortex sessions from the active Tokio runtime.
• Keep the Vortex runtime owner alive for the full reader stream lifetime.
• Keep the Vortex runtime owner alive while the background writer task is running.

Tests

• cargo fmt --all -- --check
• git diff --check
• cargo clippy --all-targets --workspace --features fulltext,vortex -- -D warnings
• cargo test -p paimon --all-targets --features fulltext,vortex

API and Format

No API or storage format changes.

Documentation

No documentation update required.

[JingsongLi]

+1