Return P2WSH script pubkey for keyed anchor prevouts

[tnull]

AnchorDescriptor::previous_utxo is used for coin selection and PSBT witness_utxo metadata. For keyed anchors it should describe the on-chain P2WSH anchor output instead of the witness script so wallets can validate and sign the package.

Co-Authored-By: HAL 9000

This finding was discovered by Project Loupe

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I've reviewed the change. It's a focused bug fix: previous_utxo() for keyed anchors previously returned the bare redeemscript as the script_pubkey, but the actual on-chain anchor output is a P2WSH (confirmed at chan_utils.rs:2343 and onchaintx.rs:710). The .to_p2wsh() addition makes the witness_utxo metadata and coin-selection prevout match the real output being spent. The anchor signature is computed separately via tx_input_witness/sign_holder_keyed_anchor_input over the redeemscript, so this doesn't affect signing.

The test uses holder_pubkeys.funding_pubkey, which matches as_holder_broadcastable().broadcaster_pubkeys().funding_pubkey used internally, and follows existing test patterns.

No issues found.

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