test: re-add temporary public function for semver testing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rluvaton

datafusion/common-runtime/src/lib.rs

@@ -33,4 +33,9 @@ pub use common::SpawnedTask;
 pub use join_set::JoinSet;
 pub use trace_utils::{
     JoinSetTracer, JoinSetTracerError, set_join_set_tracer, trace_block, trace_future,
-};
+};
+
+/// Temporary public function to test semver breaking change detection.
+pub fn semver_test_function() -> &'static str {
+    "this will be removed to trigger a breaking change"
+}