feat(python): pass merge_insert sources as scannables

Python's `execute_merge_insert` now takes a `PyScannable` (like `add()`),
and the source collection threshold is exposed as a
`source_collect_threshold_bytes` kwarg on `execute()` and a
`merge_insert_source_collect_threshold_bytes` connection option.

Node passes the decoded IPC batches as a `Vec<RecordBatch>`, so the
merge join sees their exact row count.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

nodejs/__test__/table.test.ts

@@ -1081,6 +1081,16 @@ describe("merge insert", () => {
     expect(res).toEqual(expected);
   });
 
+  test("empty source with delete by source", async () => {
+    const empty = arrow.makeEmptyTable(await table.schema());
+    const res = await table
+      .mergeInsert("a")
+      .whenNotMatchedBySourceDelete()
+      .execute(empty);
+    expect(res.numDeletedRows).toBe(3);
+    expect(await table.countRows()).toBe(0);
+  });
+
   test("timeout", async () => {
     const newData = [
       { a: 2, b: "x" },

nodejs/src/merge.rs

@@ -3,6 +3,7 @@
 
 use std::time::Duration;
 
+use arrow_array::RecordBatch;
 use lancedb::{ipc::ipc_file_to_batches, table::merge::MergeInsertBuilder};
 use napi::bindgen_prelude::*;
 use napi_derive::napi;
@@ -66,9 +67,26 @@ impl NativeMergeInsertBuilder {
 
     #[napi(catch_unwind)]
     pub async fn execute(&self, buf: Buffer) -> napi::Result<MergeResult> {
-        let data = ipc_file_to_batches(buf.to_vec()).map_err(|e| {
+        let reader = ipc_file_to_batches(buf.to_vec()).map_err(|e| {
             napi::Error::from_reason(format!("Failed to read IPC file: {}", convert_error(&e)))
         })?;
+        let schema = reader.schema();
+        // Decoded batches are rescannable, so the merge join sees their exact
+        // row count.
+        let mut data = reader
+            .map(|batch| {
+                batch.map_err(|e| {
+                    napi::Error::from_reason(format!(
+                        "Failed to read record batch from IPC file: {}",
+                        e
+                    ))
+                })
+            })
+            .collect::<Result<Vec<_>>>()?;
+        // An empty Vec has no schema, so keep the source schema in an empty batch.
+        if data.is_empty() {
+            data.push(RecordBatch::new_empty(schema));
+        }
 
         let this = self.clone();