Start issue suggestions for a project

Starts issue-suggestions generation for the confirmed repository, or returns the existing pending or finished result for the same repository, model and generation version. Returns promptly with 200 and pending while generation runs in the background: poll the GET. Calling again while a result is pending or finished starts nothing; calling after failed retries. A repository the Tessl App cannot read is recorded as a failed result with the reason in detail, not as an error here. Returns 409 when the shared cache is unavailable, because generation needs it for its lease. Requires edit access to the project workspace.