Fix `sp-api` handling of multiple arguments (#6484)

With the switch to `decode_all_with_depth_limit` we silently broken
support for functions with multiple arguments. The old generated code
tried to decode each parameter separately, which does not play well with
`decode_all`.

This pr adds a test to ensure that this does not happen again and fixes
the bug by decoding everything at once by wrapping it into tuples.