[ty] Add virtual files to the only project database (#19322)
## Summary
Previously, the virtual files were being added to the default database
that's present on the session. This is wrong because the default
database is for any files that don't belong to any project i.e., they're
outside of any projects managed by the server. Virtual files are neither
part of the project nor it is outside the projects. This was not the
intention as in the initial version, virtual files were being added to
the only project database managed by the server.
This PR fixes this by reverting back to the original behavior where
virtual files will be added to the only project database present. When
support for multiple workspace and project is added, this will require
updating (https://github.com/astral-sh/ty/issues/794).
This is required for #19264 because workspace diagnostics doesn't check
the default project database yet. Ideally, the default db should be
checked as well.
The implementation of this PR means that virtual files are now being
included for workspace diagnostics but it doesn't work completely e.g.,
if I save an untitled file the diagnostics disappears but it doesn't
appear back for the (now) saved file on disk as shown in the following
video demonstration:
https://github.com/user-attachments/assets/123e8d20-1e95-4c7d-b7eb-eb65be8c476e
