discovery: fix bug that can lead to sending invalid chan_ann msgs

[Roasbeef]

Initially in lnd, we didn't store the extra TLV data that could be dangling off of gossip messages. This was fixed initially in lnd v0.5 with this PR: #1825.

Within the PR, we incorrectly set the ExtraOpaqueData (extra TLV blob) of the ChannelAnnouncement to the value stored in edge, which is actually our channel update. As 6-ish years ago we didn't yet have anything that used the TLV gossip fields, this went unnoticed.

Fast forward to 2024, we shipped an experimental version of inbound fee discounts. This starts to store additional data in the ExtraOpaqueData field: the TLV for the inbound fee discount. Initially, everything is valid when the first ChannelAnnouncement is sent, but as soon as a user attempts to set an inbound fee policy, we'd incorrectly swap in that new serialized TLV for the channel announcement:
841e243#diff-1eda595bbebe495bd74a6a0431c46b66cb4e8b53beb311067c010feac2665dcbR2560.

Since we're just trying to generate a new channel_update, we don't also regenerate the signature for the channel_announcement message. As a result, we end up storing a channel_announcement with an invalid sig on disk, continuing to broadcast that to peers.

Fixes #9000

[coderabbitai]

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[bitromortac]

LGTM 🎉

[yyforyongyu]

LGTM🙏 Wonder what happens to the existing nodes - I guess a restart should be enough to fix the issue for them.

[yyforyongyu]

Cool think we need to do a new rc?

[Roasbeef]

Yeah, I think we have another PR or two incoming to trigger a new rc.

[Crypt-iQ]

lgtm 🌮

[Roasbeef]

Wonder what happens to the existing nodes - I guess a restart should be enough to fix the issue for them.

Reading the code diff again, we don't update the chan ann on disk to be invalid, just when we were making the wire message, we swapped in the wrong TLV data. So the chan ann's on disk are valid, and now with this PR as soon as we go to update the chan upds (zombie prevention or updatechanpolicy) we'll send out the new valid announcement.