fix(types): use collection field in UnionSearchResponseRequestParams

[rachit367]

Fixes #321.

UnionSearchResponseRequestParams extends SearchResponseRequestParams, which declares collection_name?: string. But for a union multi-search, the server returns the per-search entry under union_request_params with the field named collection (matching the field name used in the request), not collection_name. The current type forces callers to widen or cast every time they read union_request_params[i].collection.

Override the field on the union variant: omit collection_name from the inherited shape and add collection: string. Regular SearchResponseRequestParams (used by SearchResponse.request_params) is unchanged.

Confirmed against the response shape from the issue:

{
  "union_request_params": [
    { "collection": "collection_one", "first_q": "test", "found": 30, ... }
  ]
}

No runtime code touched; this is types-only.

[tharropoulos]

Which version of Typesense was this tested on? A PR to return collection_name (to match the rest of the search request responses) was added to Typesense on August 2025: typesense/typesense#2473.

This has been the case for all major Typesense versions after 29.0.

[tharropoulos]

Please leave a TODO comment underneath the collection prop, we should deprecate this.

[tharropoulos]

Maybe remove the omit, keep it as an optional prop and add collection as optional as well