Support for GHC versions that do not have Stackage snapshots

I would like to summon GHC 9.6.2 projects, but that is not possible yet. So, I've quickly looked into the source code and what kind of changes that would require. Adding the metadata seems simple for the most part, except that each GHC version seems to need a `latestLts` Stackage snapshot associated with it.

I could change `latestLts` to return a `Maybe Text` rather than just `Text` to represent missing Stackage snapshots. But that raises the question what should happen when Stackage versions are missing. I guess the `script` command could simply show an error message, but the problem of summoning a project with multiple GHC versions of which one is not supported seems more difficult to solve. We could silently leave that version out, show a warning, or refuse to summon the whole project with an error.

What would be the best way to resolve this?

Start by tracing how GHC metadata, latestLts, and the script command handle Stackage snapshots, then inspect the summon path for projects with multiple GHC versions. Define and document the behavior for missing snapshots, including whether a single unsupported version should warn, be omitted, or abort; done means GHC 9.6.2 projects and mixed-version projects have a consistent outcome.

Written by the indexing model from the issue text.