Improve Zoekt API code and specs

The following discussions from !116650 (merged) should be addressed:

@splattael started a discussion: (+2 comments)

Question Since this feature is behind two feature flags (#388519) do we want to add a check if zoekt is enabled for the passed project/namespace? 🤔
@splattael started a discussion:
Suggestion Thoughts on using non_existing_record_id instead?
```
      let(:project_id) { non_existing_record_id }
```
See https://docs.gitlab.com/ee/development/testing_guide/best_practices.html#general-guidelines

      let(:shard_id) { non_existing_record_id }

        let(:shard_id) { non_existing_record_id }

        let(:namespace_id) { non_existing_record_id }

        let(:shard_id) { non_existing_record_id }

        let(:namespace_id) { non_existing_record_id }

Edited May 08, 2023 by Dylan Griffith