Issue field to identify if it is a build, boot or test issue

Today, one can only understand this if you query issues and check for build_id and test_id to NULL. It would be great to know that right away. I see the issue type as a very important information to have readily available.

Start by tracing how issues are represented and queried, focusing on the existing build_id and test_id fields and how their NULL values distinguish issue categories. Define how the new field identifies build, boot, or test issues, then add coverage showing that the classification is available without inspecting those identifiers directly.

Written by the indexing model from the issue text.