`MultiSearchResponse` type does not account for union search response shape

[rabingaire]

The MultiSearchResponse type in src/typesense/types/multi_search.py only models the federated (default) multi-search response shape:

class MultiSearchResponse(typing.TypedDict):
    results: typing.List[SearchResponse[typing.Any]]

However, the /multi_search endpoint has two distinct response shapes depending on whether union is set in the request:

• Federated search (union absent or False): returns { "results": [ ...one SearchResponse per query... ] }
• Union search (union: True): returns with no results wrapper doc

The current type is incorrect for the union case and gives users a misleading return type annotation when they call multi_search.perform() with union=True.