feat: add PerformUnion to MultiSearch to properly handle Union queries

[haydenhoang]

Change Summary

This PR adds a dedicated PerformUnion method to MultiSearchInterface. When performing a multi-search with Union: true, the API returns a single SearchResult rather than an array of results. The existing Perform method attempts to parse this into a MultiSearchResult, which does not map correctly. This fixes #226.

Changes:

• Added PerformUnion which forces Union: true on the request body and parses the response directly into api.SearchResult.
• Added fail-fast validation to reject requests where a user explicitly passes Union: false to PerformUnion.
• Added unit and integration tests.

Notes on OpenAPI Spec:

The last PR #216 adding support for union search added the hits field to MultiSearchResult in the local OpenAPI spec file but not in the shared Typesense API Spec. So a simple go generate ./... will override that PR's change.

I have chosen not to run go generate ./... in this PR, as pulling the latest spec introduces a lot of unrelated breaking changes to the Go client that should be handled in a separate major update.

PR Checklist

• I have read and signed the Contributor License Agreement.

[haydenhoang]

@kishorenc