[ty] add benchmark for large union type narrowing (#23546)
## Summary
From https://github.com/astral-sh/ruff/pull/23201#discussion_r2841921540
Let's merge this benchmark into main ahead of #23201 so that we can see
the impact of the changes.
## Test Plan
`benchmark_large_union_narrowing` added to `ruff_benchmark/ty.rs`
