wallet: harden external signer psbt processing, revamp mock #36114

`PSBTInput::Merge` copies every optional input field from the other
input when it is absent locally, except `PSBT_IN_SIGHASH_TYPE`. So
`combinepsbt` silently drops the sighash type whenever the first PSBT
does not carry it, making the result depend on the argument order.

The field is what lets finalizers enforce the sighash type of existing
signatures (BIP 174). When it is lost, `FinalizePSBT` falls back to the
default type (`SIGHASH_ALL`, or `SIGHASH_DEFAULT` for taproot inputs),
rejects signatures made with any other type as a sighash mismatch, and
the PSBT can no longer be finalized. Combining a PSBT signed with
`ALL|ANYONECANPAY` after a merely updated copy of the same PSBT
reproduces this: `finalizepsbt` reports it as incomplete, while the
reverse order finalizes and broadcasts fine.

Merge the sighash type like the other optional fields, keeping the one
already present, and test both combine orders.

The PSBT sighash type field is a 32 bit unsigned integer in BIP 174,
signed in PSBTInput, and it is not validated when deserialized.

decodepsbt incorrectly truncates this field before looking up its
name. Fix that and add a test.

The external signer mock previously replayed a PSBT that the test
prepared in advance. This makes it difficult to test more complicated
scenarios like a misbehaving wallet and (MuSig2) multisig.

Instead, give the mock its own descriptor wallet. The test provides a
dedicated node for this wallet and keeps it offline, so the mock can't
cheat by e.g. inspecting the UTXO set. The mock creates the wallet on
first use and signs with walletprocesspsbt.

wallet_signer.py now funds all four descriptor types and spends them
in a single transaction, exercising every signing code path.

Previously the PSBT returned by the external signer replaced the
original wholesale, trusting the signer not to modify the transaction.
Merge it instead. This rejects a response that describes a different
transaction.

Merging also supports signers that strip fields they don't need from
their response. New tests cover both scenarios.

Co-authored-by: brunoerg <brunoely.gc@gmail.com>

A signature with SIGHASH_NONE or SIGHASH_SINGLE doesn't commit to all
outputs, letting anyone alter them after signing. Reject a PSBT from
an external signer that declares such a sighash type or contains
signatures made with one.

SIGHASH_ANYONECANPAY is still accepted: it only permits adding inputs,
which does not affect us.

The mock signer produces real signatures for these scenarios by
letting its wallet sign with the requested sighash type, optionally
hiding the declared sighash type field so that only the signatures
themselves reveal it.

Co-authored-by: brunoerg <brunoely.gc@gmail.com>