[keyserver] Validate search endpoint response

Summary:
issue: https://linear.app/comm/issue/ENG-3939/validate-search-endpoint
Search endpoint was missing a validation on the response

Test Plan: Called the search endpoint from client, checked that no errors / warnings appear

Reviewers: michal

Reviewed By: michal

Subscribers: atul, ashoat, tomek

Differential Revision: https://phab.comm.dev/D8143