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>

nodejs/src/table.rs

@@ -165,7 +165,7 @@ impl Table {
         if let Some(train) = train {
             builder = builder.train(train);
         }
-        builder.execute().await.default_error()
+        builder.execute().await.default_error().map(|_| ())
     }
 
     #[napi(catch_unwind)]