Fixes #32849 descriptorprocesspsbt crashes with an internal bug assertion when given a PSBT whose inputs looked “finalized” that is non-empty final_script_sig / witness but whose signatures did not verify like invalid Schnorr bytes with unusual sighash combinations such as SIGHASH_SINGLE | ANYONECANPAY.
Completeness was inferred with PSBTInputSigned, which only checks that final fields are present, not that they pass script verification. That let the RPC treat the PSBT as complete and call FinalizeAndExtractPSBT, where a failing invariant surfaced as CHECK_NONFATAL instead of a normal incomplete outcome.
This PR determines complete the same way as the wallet path: after ProcessPSBT, it recomputes PrecomputedTransactionData and sets complete only if every input passes PSBTInputSignedAndVerified. Invalid signatures then yield complete: false and no hex, without going through the finalize path that asserted.