[bug]: responder of taproot assets channel doesn't have proof for commitment transaction

[guggero]

Found while adding more assertions to the custom channel integration tests in litd: lightninglabs/lightning-terminal#784

If the responder of a channel initiates the co-op close process, they seem to not have the proofs for the commitment transaction available in the local proof archive.
This results in the following error:

2024-06-28 17:50:31.085 [ERR] FRTR: Error evaluating state (SendStateStoreProofs): failed to generate an updated proof file for output 0: error fetching input proof: error fetching input proof -- locator=(proof.Locator) {
 AssetID: (*asset.ID)(0xc001034fe4)((len=32 cap=32) 452d67bfd17372691f86c3410ec836fae677f5a2119097c50680d463a2c58b1f),
 GroupKey: (*secp256k1.PublicKey)(<nil>),
 ScriptKey: (secp256k1.PublicKey) {
  x: (secp256k1.FieldVal) 50aaeb166f4234650d84a2d8a130987aeaf6950206e0905401ee74ff3f8d18e6,
  y: (secp256k1.FieldVal) 8e5b090f56a33ca5081a001b402f774ebd0690abed9273a47dec5c3ae92d9786
 },
 OutPoint: (*wire.OutPoint)(0xc001034fc0)(c26984a76cec3f420e59cef8f8faa8057c5f382cad999b7c9861219095d7e33b:0)
}
: unable to find proof

That's why the transfer then never fully completes and the final balance isn't correct.

[Roasbeef]

I think we need to call auxSweeper.importOutputProof when the co-op close process starts. Better yet, with a slightly larger change, we can hook into the chanvalidate module for lnd, and make sure that once the funding transaction is confirmed, all the proofs are imported.