psbt: support output metadata updates before inputs are added #35797

Document the scripts and key origins collected for common output descriptors before changing zero-input handling.
Use a valid input for ECDSA-backed outputs because their zero-input paths currently abort.

PSBTv2 permits outputs to be added before inputs, but `UpdatePSBTOutput()` traverses output scripts with a signature creator for input 0.
A descriptor with private key material can therefore reach `SignatureHash()` without a corresponding input and abort.
Reject the signature attempt while preserving the scripts and key origins collected during traversal.

Add a relative-timelock case that consults the transaction checker during output metadata traversal.
Use a valid input because the pre-fix missing-input path is only reliably diagnosed under a sanitizer.

A miniscript relative timelock can consult `CheckSequence()` while `UpdatePSBTOutput()` is only collecting metadata.
Return a rejecting checker when the creator has no corresponding input, so traversal can continue without reading a missing sequence.

Document direct Taproot signing with a valid input before covering a missing input index.
The PSBT output-update path has no precomputed transaction data, so it cannot exercise this sighash path.

`ComputeSchnorrSignatureHash()` is shared by Schnorr signing and MuSig2 operations and can pass a missing input index to `SignatureHashSchnorr()`.
Reject the request before computing the sighash, matching the missing-input behavior for ECDSA signing and transaction checks.