Only include personal snippets in snippets API

Summary

The snippets API doesn't exclude project snippets, but uses Entities::PersonalSnippet to present them and checks for *_personal_snippet permissions in some places, so this looks like a bug.

For the /snippets/public endpoint we're fixing this as part of https://gitlab.com/gitlab-org/gitlab/issues/35389.

Improvements

Make sure all endpoints only interact with personal snippets.

Risks

The API docs don't mention personal and project snippets, so we might break somebody's workflow.

We might first want to introduce a proper API for project snippets before changing this. We could do that on top of our planned GraphQL API: #36079 (closed)

Involved components

• lib/api/snippets.rb