fix(diagnostic): don't convert diagnostic table twice

The recursive implementation of vim.lsp.diagnostic.get() applied
`diagnostic_vim_to_lsp` twice, and the second time gave wrong
results because of the unexpected format.

Fixes https://github.com/neovim/neovim/issues/15689

runtime/lua/vim/lsp/diagnostic.lua

@@ -265,7 +265,7 @@ function M.get(bufnr, client_id, predicate)
         table.insert(all_diagnostics, diagnostic)
       end
     end)
-    return diagnostic_vim_to_lsp(all_diagnostics)
+    return all_diagnostics
   end
 
   local namespace = M.get_namespace(client_id)