rpc: analyzepsbt should explicitly check for an invalid taproot_key_path_sig #33320

While testing #29675 my final combined PSBT ended up having an invalid taproot_key_path_sig, which I found out by logging the sigdata.complete value at the end of ProduceSignature.

We should of course never produce such an invalid value and I'll continue digging for the root cause. But external applications could do this too.

Currently analyzepsbt will give a useless "next": "updater" status when this happens.

We should instead display an error.

Not sure what the best approach would be.

FWIW this particular situation happened because a ledger signed one of the script paths and my WIP branch for HWI ended up putting that signature in the key path.

HWI misclassified a script-path signature, so src/node/psbt.cpp::AnalyzePSBT sees an invalid key-path signature and falls back to UPDATER.

I don’t think this requires a fix: UPDATER is the fallback for incomplete inputs that cannot be classified as merely missing signatures. Changing this logic would require additional failure states and significantly complicate the analysis.

ref: https://github.com/bitcoin-core/HWI/commit/fc206450e71cf07b2211a06d606ca7567bcc9170