Fix querying vulnerability_reads by cluster_agent_id

What does this MR do and why?

When adding vulnerability_reads.cluster_agent_id column, we have incorrectly set it as a TEXT column instead of BIGINT. It has some implications that now we have to mitigate with MRs: Change column type of vulnerability_reads.clust... (!88333 - merged) and Add background migration to backfill casted_clu... (!89915 - merged).

With this MR we are temporarily fixing filtering vulnerabilities by cluster_agent_id. We already have indexes in place (index_vulnerability_reads_on_cluster_agent_id).

MR acceptance checklist

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