feat: add use_index parameter to merge insert operations (#2674)

## Summary

Exposes `use_index` Merge Insert parameter, which was created upstream
in https://github.com/lancedb/lance/pull/4688.

## API Examples

### Python
```python
# Force table scan
table.merge_insert(["id"]) \
    .when_not_matched_insert_all() \
    .use_index(False) \
    .execute(data)
```

### Node.js/TypeScript
```typescript
// Force table scan  
await table.mergeInsert("id")
    .whenNotMatchedInsertAll()
    .useIndex(false)
    .execute(data);
```

### Rust
```rust
// Force table scan
let mut builder = table.merge_insert(&["id"]);
builder.when_not_matched_insert_all()
       .use_index(false);
builder.execute(data).await?;
```

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

nodejs/lancedb/merge.ts

@@ -70,6 +70,23 @@ export class MergeInsertBuilder {
       this.#schema,
     );
   }
+
+  /**
+   * Controls whether to use indexes for the merge operation.
+   *
+   * When set to `true` (the default), the operation will use an index if available
+   * on the join key for improved performance. When set to `false`, it forces a full
+   * table scan even if an index exists. This can be useful for benchmarking or when
+   * the query optimizer chooses a suboptimal path.
+   *
+   * @param useIndex - Whether to use indices for the merge operation. Defaults to `true`.
+   */
+  useIndex(useIndex: boolean): MergeInsertBuilder {
+    return new MergeInsertBuilder(
+      this.#native.useIndex(useIndex),
+      this.#schema,
+    );
+  }
   /**
    * Executes the merge insert operation
    *