Failure to reject `option_scid_alias` on an announced channel

BOLT 2 forbids the sender of `open_channel` from setting the `option_scid_alias` bit in `channel_type` when `announce_channel` is `true`:

> The sending node:
> - if it includes `open_channel_tlvs`:
> - MUST set `channel_type`:
> - if `announce_channel` is `true` (not `0`):
> - MUST NOT send `channel_type` with the `option_scid_alias` bit set.

CLN never generates this combination as opener, but it does not reject it on receipt: `channel_type_accept()` never sees `channel_flags`, so the fundee happily replies with `accept_channel` echoing back `option_scid_alias` on a channel it will then announce. The spec does not specify rules for the acceptor in this case, so accepting it is not itself a violation, but the resulting channel is internally inconsistent.

### Impact

Both `openingd` and `dualopend` are affected. The effect is that both can exchange `announcement_signatures` and publicly announce the channel using its real `short_channel_id`. However, per BOLT, when the agreed channel type has `option_scid_alias` set, a node MUST NOT allow incoming HTLCs to that channel using the real `short_channel_id`. So if the peer returns its signatures, CLN ends up announcing a channel to the whole network and then refusing every HTLC (addressed to that `short_channel_id`) it is asked to forward to the peer, which only affects the peer.

Only forwards towards the peer are affected, inbound HTLCs still work. For CLN, this means gaining some bad reputation for unnecessarily failing HTLCs. Other than that, this is purely a spec-compliance issue.

### Discovery
This bug was found while fuzzing the v1 funding protocol with [smite](https://github.com/lnfuzz/smite).

Start at channel_type_accept() and trace how channel_flags are passed through the openingd and dualopend funding paths. Reproduce the v1 funding protocol case where an announced open_channel sets option_scid_alias, using the smite fuzzing discovery as context. Done means the invalid combination is rejected on receipt while valid channel types continue through the protocol.

Written by the indexing model from the issue text.