fix(node): preserve schemas for empty query results (#4423)

Empty local TypeScript queries returned a zero-row Arrow table with no
columns because `toArrow()` built the table solely from record batches.
Local streams can yield no batches, while remote streams often supply an
empty batch carrying the schema.

Expose the executed stream's schema through the native iterator and
decode it into an empty Arrow table when no batches arrive. This
preserves projected and search columns, types, nullability, and metadata
without executing another query. Document the empty-result behavior.

Validation:

- Eleven regression cases failed with the original `toArrow()`
implementation; all twelve now pass, including remote schema-only and
empty-batch responses.
- All 1,075 tests in the query, remote, Arrow, and table suites pass.
- Native build, TypeScript compilation, Rust formatting and Clippy (`-D
warnings`), TypeScript lint/formatting, and documentation generation
pass.

Fixes #4386

<!-- lance-gatekeeper-fix:v1 agent=7d589da1f7db5c7383f9600490e111a9
generation=1 -->

Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>

nodejs/src/iterator.rs

@@ -7,6 +7,8 @@ use lancedb::ipc::batches_to_ipc_file;
 use napi::bindgen_prelude::*;
 use napi_derive::napi;
 
+use crate::util::schema_to_buffer;
+
 /** Typescript-style Async Iterator over RecordBatches */
 #[napi]
 pub struct RecordBatchIterator {
@@ -19,6 +21,12 @@ impl RecordBatchIterator {
         Self { inner }
     }
 
+    /// Return the executed stream's output schema as an Arrow IPC file.
+    #[napi]
+    pub fn schema(&self) -> napi::Result<Buffer> {
+        schema_to_buffer(&self.inner.schema())
+    }
+
     #[napi(catch_unwind)]
     pub async unsafe fn next(&mut self) -> napi::Result<Option<Buffer>> {
         if let Some(rst) = self.inner.next().await {