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>

python/python/lancedb/merge.py

@@ -33,6 +33,7 @@ class LanceMergeInsertBuilder(object):
         self._when_not_matched_by_source_delete = False
         self._when_not_matched_by_source_condition = None
         self._timeout = None
+        self._use_index = True
 
     def when_matched_update_all(
         self, *, where: Optional[str] = None
@@ -78,6 +79,23 @@ class LanceMergeInsertBuilder(object):
             self._when_not_matched_by_source_condition = condition
         return self
 
+    def use_index(self, use_index: bool) -> LanceMergeInsertBuilder:
+        """
+        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.
+
+        Parameters
+        ----------
+        use_index: bool
+            Whether to use indices for the merge operation. Defaults to `True`.
+        """
+        self._use_index = use_index
+        return self
+
     def execute(
         self,
         new_data: DATA,

python/python/lancedb/table.py

@@ -3920,6 +3920,7 @@ class AsyncTable:
                 when_not_matched_by_source_delete=merge._when_not_matched_by_source_delete,
                 when_not_matched_by_source_condition=merge._when_not_matched_by_source_condition,
                 timeout=merge._timeout,
+                use_index=merge._use_index,
             ),
         )