tor-proto: Prevent congestion control extension during ntor-v3 extend

This was missed in commits ccb65961 and eeda643f. While params.ccontrol.is_enabled() should always be false because of those earlier commits which ensure we don't enable congestion control, we were missing the defense-in-depth conditions here that would alert us if we accidentally did enable congestion control.

This now matches the code at https://gitlab.torproject.org/tpo/core/arti/-/blob/5f8ce63fbb2d3a095b09629eb77df953a0901ab0/crates/tor-proto/src/tunnel/reactor/circuit.rs#L962-L991.

This is a good motivation for #1969 (closed).

/cc @nickm