Fix resource leak: close staging Rows in execStagingOperation (#325)

## Summary
- `execStagingOperation` creates a `Rows` object via `rows.NewRows()` to
read staging operation metadata (presigned URL, headers, local file
path) but never calls `row.Close()`
- This leaks the `Rows` object and its `RowScanner` resources until GC
collects them
- Add `defer row.Close()` to ensure proper cleanup after reading the
staging metadata

Note: the server-side operation is already closed by `ExecContext`
(lines 122-133), so this is a client-side resource leak rather than a
server-side operation leak.

## Test plan
- [x] Existing `TestConn_execStagingOperation` tests pass
- [x] `TestWorkflowExample` e2e test passes
- [x] `go build ./...` compiles cleanly

Related: #275

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

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: vikrantpuppala

connection.go

@@ -592,6 +592,7 @@ func (c *conn) execStagingOperation(
 		if err != nil {
 			return dbsqlerrint.NewDriverError(ctx, "error reading row.", err)
 		}
+		defer row.Close()
 
 	} else {
 		return dbsqlerrint.NewDriverError(ctx, "staging ctx must be provided.", nil)