psbt: drop Taproot and MuSig2 signatures when adding an input #36567

PartiallySignedTransaction::AddInput clears partial_sigs, final_script_sig and final_script_witness of the input it adds, because the transaction is about to change and those signatures would no longer be valid. The Taproot key and script signatures and the MuSig2 pubnonces and partial signatures were left in place, so joinpsbts handed back inputs carrying signatures for the old transaction.

They are cleared in the same place now.

The functional test joins two PSBTs with signed Taproot inputs: before the change the joined inputs still have taproot_key_path_sig, after it they don't, and the wallet can sign the joined PSBT. It fails on master at that assertion. rpc_psbt.py and the psbt unit tests pass.

Concept ACK b40ac85. With this PR, the v0 branch of AddInput clears every field that PSBTInput::HasSignatures checks, plus the MuSig2 public nonces. btclib-wallet, which I maintain, clears the same seven fields when it joins v0 PSBTs. I ran this PR's test scenario through it: no joined input keeps a key-path signature, and the joined PSBT signs both inputs again.

ACK b40ac85ff5df1f3a32912a990bddfffed3a64cb7

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36567
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36567.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36567#author-ralph

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #36521 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/36521.svg">} (Keep witness stacks in serialized form in CScriptWitness by sipa)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

Tested ACK b40ac85ff5 on Windows.