test(rust): cover fixed-size-list merge overflow (#3907)

## Summary

- add a merge-insert regression test whose fixed-size-list child count
crosses `u32::MAX`
- verify delete-by-source updates the matching row, deletes every other
row, and completes without an Arrow panic
- use a null child array so the boundary case avoids allocating a real
vector payload

## Root cause and fix

The affected Lance merge fallback carried the target payload through a
full outer hash join. Arrow's fixed-size-list take kernel uses `u32`
child indices, so taking a target row whose child offset crossed
`u32::MAX` wrapped the offset and produced child data shorter than the
parent array, triggering the reported `ArrayData::slice` assertion.

The projection-aware merge path in the Lance version now used by `main`
avoids materializing the target fixed-size-list payload in that join.
This regression test locks in that production behavior at the exact
child-index boundary.

## Validation

- `cargo fmt --all`
- `cargo test --quiet --features remote -p lancedb
test_merge_insert_fixed_size_list_above_u32_child_count`
- `cargo check --quiet --features remote --tests --examples`
- `cargo clippy --quiet --features remote --tests --examples`

Fixes #2874

<!-- lance-gatekeeper-fix:v1 agent=582e68bcad65739e189352cb3cbf144c
generation=3 -->

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

rust/lancedb/src/table/merge.rs

@@ -321,7 +321,8 @@ pub(crate) async fn execute_merge_insert(
 mod tests {
     use arrow_array::builder::FixedSizeBinaryBuilder;
     use arrow_array::{
-        Int32Array, RecordBatch, RecordBatchIterator, RecordBatchReader, StringArray, UInt64Array,
+        FixedSizeListArray, Int32Array, NullArray, RecordBatch, RecordBatchIterator,
+        RecordBatchReader, StringArray, UInt32Array, UInt64Array,
     };
     use arrow_schema::{DataType, Field, Schema};
     use std::sync::Arc;
@@ -529,6 +530,74 @@ mod tests {
         assert_eq!(result.num_deleted_rows, 5);
         assert_eq!(table.count_rows(None).await.unwrap(), 5);
     }
+
+    #[tokio::test]
+    async fn test_merge_insert_fixed_size_list_above_u32_child_count() {
+        // Arrow's FixedSizeList take kernel uses u32 child indices. Previously,
+        // delete-by-source materialized the target payload in a full outer join,
+        // causing the final list below to overflow those indices and panic.
+        // A Null child keeps this boundary test small in memory.
+        const LIST_SIZE: i32 = 65_536;
+        const ROW_COUNT: usize = (u32::MAX as usize / LIST_SIZE as usize) + 1;
+        const BATCH_SIZE: usize = 8_192;
+
+        let item = Arc::new(Field::new("item", DataType::Null, true));
+        let schema = Arc::new(Schema::new(vec![
+            Field::new("id", DataType::UInt32, false),
+            Field::new(
+                "vector",
+                DataType::FixedSizeList(item.clone(), LIST_SIZE),
+                false,
+            ),
+        ]));
+        let batch = |start: usize, len: usize| {
+            RecordBatch::try_new(
+                schema.clone(),
+                vec![
+                    Arc::new(UInt32Array::from_iter_values(
+                        start as u32..(start + len) as u32,
+                    )),
+                    Arc::new(FixedSizeListArray::new(
+                        item.clone(),
+                        LIST_SIZE,
+                        Arc::new(NullArray::new(len * LIST_SIZE as usize)),
+                        None,
+                    )),
+                ],
+            )
+            .unwrap()
+        };
+
+        let target_batches = (0..ROW_COUNT)
+            .step_by(BATCH_SIZE)
+            .map(|start| {
+                let len = (ROW_COUNT - start).min(BATCH_SIZE);
+                Ok(batch(start, len))
+            })
+            .collect::<Vec<_>>();
+        let target_data: Box<dyn RecordBatchReader + Send> =
+            Box::new(RecordBatchIterator::new(target_batches, schema.clone()));
+        let conn = connect("memory://").execute().await.unwrap();
+        let table = conn
+            .create_table("fixed_size_list_overflow", target_data)
+            .execute()
+            .await
+            .unwrap();
+
+        let source = batch(ROW_COUNT - 1, 1);
+        let mut merge = table.merge_insert(&["id"]);
+        merge
+            .when_matched_update_all(None)
+            .when_not_matched_by_source_delete(None);
+        let result = merge
+            .execute(Box::new(RecordBatchIterator::new([Ok(source)], schema)))
+            .await
+            .unwrap();
+
+        assert_eq!(result.num_updated_rows, 1);
+        assert_eq!(result.num_deleted_rows, (ROW_COUNT - 1) as u64);
+        assert_eq!(table.count_rows(None).await.unwrap(), 1);
+    }
 }
 
 #[cfg(test)]