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/lancedb/query.ts

@@ -41,26 +41,6 @@ export async function* RecordBatchIterator(
   }
 }
 
-class RecordBatchIterable<
-  NativeQueryType extends NativeQuery | NativeVectorQuery | NativeTakeQuery,
-> implements AsyncIterable<RecordBatch>
-{
-  private inner: NativeQueryType;
-  private options?: QueryExecutionOptions;
-
-  constructor(inner: NativeQueryType, options?: QueryExecutionOptions) {
-    this.inner = inner;
-    this.options = options;
-  }
-
-  // biome-ignore lint/suspicious/noExplicitAny: skip
-  [Symbol.asyncIterator](): AsyncIterator<RecordBatch<any>, any, undefined> {
-    return RecordBatchIterator(
-      this.inner.execute(this.options?.maxBatchLength, this.options?.timeoutMs),
-    );
-  }
-}
-
 /**
  * Options that control the behavior of a particular query execution
  */
@@ -271,13 +251,20 @@ export class QueryBase<
     return RecordBatchIterator(this.nativeExecute());
   }
 
-  /** Collect the results as an Arrow @see {@link ArrowTable}. */
+  /**
+   * Collect the results as an Arrow @see {@link ArrowTable}.
+   *
+   * Empty results retain the query's output schema.
+   */
   async toArrow(options?: Partial<QueryExecutionOptions>): Promise<ArrowTable> {
     const batches = [];
-    const inner = await this.getInner();
-    for await (const batch of new RecordBatchIterable(inner, options)) {
+    const iterator = this.nativeExecute(options);
+    for await (const batch of RecordBatchIterator(iterator)) {
       batches.push(batch);
     }
+    if (batches.length === 0) {
+      return tableFromIPC((await iterator).schema());
+    }
     return new ArrowTable(batches);
   }