rust: create_index returns the server-minted job id

BaseTable::create_index and IndexBuilder::execute now return
Option<String> -- the job id the server mints when an index build is
deferred to a background job (pending vector index on a remote table).
Native builds are synchronous and return None; older servers with empty
response bodies parse as None. The pyo3 binding forwards the id to
python; nodejs keeps its unit return.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

python/src/table.rs

@@ -794,8 +794,8 @@ impl Table {
         }
 
         future_into_py(self_.py(), async move {
-            op.execute().await.infer_error()?;
-            Ok(())
+            let job_id = op.execute().await.infer_error()?;
+            Ok(job_id)
         })
     }