wallet: walletprocesspsbt does not add input bip32_derivs when signing #18039

Steps to reproduce:

1. Create PSBT without BIP32 derivation info: bitcoin-cli -testnet walletcreatefundedpsbt '[]' '[{"mkHS9ne12qx9pS9VojpwU5xtRd4T7X7ZUt": 0.001}]' 0 '{"feeRate": 0.0001}' false

2. Process, but don't sign: walletprocesspsbt THE_PSBT false ALL true

Notice that this PSBT has bip32_derivs set for all inputs.

2. Process and sign: walletprocesspsbt THE_PSBT true ALL true

Notice that this PSBT has bip32_derivs set for the change output, but not for the inputs.

cc @achow101

Unable to replicate. The bip32_derivs for the inputs in my test tx are filled.

TIL walletprocesspsbt is also a Finalizer. It's probably enough to just document the bip32_derivs argument to mention that it won't add that for complete inputs.