Stabilize async span-capture tests

[geoffreyclaude]

This pulls stabilization-only changes out of the DataFusion 54 upgrade branch so #55 can stay focused on the dependency/API update.

What Changed

• Propagate the span-capture subscriber across async poll boundaries in unit tests instead of relying on thread-local scoped defaults.
• Apply the same subscriber propagation to the stale optimizer phase-span regression test.
• Normalize duplicate recursive execution close events in the all-options recursive integration snapshot while keeping preview assertions based on the uncollapsed trace.

Scope

This does not change DataFusion dependencies or production instrumentation behavior.

Validation

• cargo fmt --all --check
• git diff --check
• INSTA_UPDATE=always cargo test -p datafusion-tracing-tests --test integration_tests test_recursive_all_options -- --exact
• cargo test -p datafusion-tracing --lib --quiet
• cargo test --workspace --quiet