Export to_datafusion_err from the util crate root (#1487)

* Re-export error helpers from crate root

Publicly re-export curated error helpers, including
to_datafusion_err, from the crate root. Add a
regression test in crates/util/tests/root_exports.rs
to ensure correct functionality in an integration-test
context.

* Restrict re-exported items in lib.rs

Make only to_datafusion_err publicly re-exported from the crate
root. Keep PyDataFusionError and PyDataFusionResult as private
imports for internal use, enhancing encapsulation and reducing
exposure of non-essential components.

* feat: remove unnecessary blank line in lib.rs to improve code formatting

* rm root_exports.rs

Author: kosiew

crates/util/src/lib.rs

@@ -36,9 +36,9 @@ use tokio::runtime::Runtime;
 use tokio::task::JoinHandle;
 use tokio::time::sleep;
 
-use crate::errors::{PyDataFusionError, PyDataFusionResult, to_datafusion_err};
-
 pub mod errors;
+pub use crate::errors::to_datafusion_err;
+use crate::errors::{PyDataFusionError, PyDataFusionResult};
 
 /// Utility to get the Tokio Runtime from Python
 #[inline]