Function resolve_context 

fn resolve_context<'a>(
    file_uri: &Url,
    root: &Path,
    project_cache: &ProjectCache,
    prefix: &'a PrefixContext<'a>,
) -> Option<CompletionContext<'a>>

Build a CompletionContext from the file URI, project cache, and prefix.

Returns None if the file is not under models/<database>/<schema>/ (i.e., the default database/schema cannot be determined from the path).