fix: preserve typed DataFusion errors through Shared and ReplayExec

[Copilot]

Two error paths erased the concrete type of a DataFusionError, retaining only its display string—making it impossible for callers to downcast_ref or walk the source chain to recover structured error information.

Changes

lance-core/src/error.rs — DataFusionError::Shared with multiple owners

When Arc::try_unwrap fails (the Arc has >1 owner), the previous fallback constructed Error::Execution(shared.to_string()), dropping the typed inner error. Now it re-wraps the Arc as DataFusionError::Shared(shared) and stores it via Error::External, so the original type remains reachable through Error::source() / downcast_ref.

lance/src/io/exec/utils.rs — ReplayExec shared stream

ShareableRecordBatchStream previously routed errors through lance_core::Error + CloneableError. On clone, CloneableError fell through to Error::cloned(to_string()) for Error::External variants, discarding the type before the second consumer ever saw it.

The stream now stores errors as Arc<DataFusionError> (which is Clone) and the adapter emits them as DataFusionError::Shared(arc). Both consumers share the same Arc and retain full access to the typed source chain.

// Before — second consumer receives a display-only string error
.map_err(|e| DataFusionError::External(e.0.to_string().into()))

// After — both consumers get DataFusionError::Shared(arc) pointing at the original typed error
.map_err(DataFusionError::Shared)

Tests

• test_datafusion_shared_multi_owner_preserves_type (lance-core): verifies a marker error carried as DataFusionError::External survives conversion from a multiply-owned DataFusionError::Shared.
• test_replay_preserves_typed_error (lance): verifies both ReplayExec partitions can downcast_ref to the original typed error.

• Fixes Preserve typed DataFusion errors through Shared and ReplayExec #8676