fix: propagate tracing context across async boundaries to prevent orphan spans

[sinianluoye]

Summary

When Lance queries are executed through external callers (e.g. NAPI bindings) with distributed tracing enabled, many internal Rust spans appear as disconnected root traces in backends like Tempo instead of nesting under the caller's span tree.

Root causes:

1. execute_plan() did not wrap the DataFusion plan with TracedExec, so lazy plan nodes captured Span::current() at poll time (often <none>) instead of at plan creation time.
2. Numerous async boundaries (boxed futures, boxed streams, spawned tasks) did not propagate the current span.

Changes:

• Add TracedExec wrapper in execute_plan() to capture and propagate the caller's span through lazy DataFusion plan execution
• Register LanceJoinSetTracer via set_join_set_tracer() so DataFusion's internal spawned tasks also carry the current span
• Add FutureTracingExt trait (future_in_current_span, boxed_in_current_span, boxed_in_span) alongside the existing StreamTracingExt in lance-core
• Add spawn_in_current_span / spawn_in_span helpers for tokio::spawn
• Convert all tracing-sensitive execution paths to use these shared helpers: decoder, scheduler, fragment reader, scan, filtered read, take, FTS, pushdown scan, cache
• Add tracing context propagation documentation

Test plan

• 7 regression tests added in rust/lance/tests/query/tracing.rs using a SpanTreeRecorder layer that asserts every span is reachable from the test root (no orphans):
  • Multi-fragment scan (memory + local filesystem)
  • FTS with filter
  • Vector search (IVF-PQ)
  • Hybrid FTS + vector reranker
  • Local filesystem variants for scan, vector, and hybrid paths
• cargo test -p lance --features slow_tests --test integration_tests query::tracing:: — 7/7 pass
• cargo fmt --all -- --check — clean
• cargo check -p lance --features slow_tests --tests — clean

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 91.56909% with 36 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-datafusion/src/exec.rs	57.69%	10 Missing and 1 partial ⚠️
rust/lance-encoding/src/decoder.rs	91.91%	4 Missing and 4 partials ⚠️
rust/lance/src/io/exec/scan.rs	80.95%	2 Missing and 6 partials ⚠️
rust/lance/src/io/exec/pushdown_scan.rs	85.71%	2 Missing and 1 partial ⚠️
rust/lance-core/src/cache/mod.rs	87.50%	0 Missing and 2 partials ⚠️
rust/lance/src/io/exec/filtered_read.rs	87.50%	2 Missing ⚠️
.../lance-encoding/src/encodings/logical/primitive.rs	80.00%	1 Missing ⚠️
rust/lance-io/src/scheduler.rs	98.11%	0 Missing and 1 partial ⚠️

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