external signer: verify PSBT is reliable after signing it #35358

Seeking Concept ACK

A compromised or "rogue" external signer could return a PSBT with silently altered outputs or use unsafe sighash types that do not commit to all outputs. This PR adds a post-signing validation step in External::SignTransaction that compares the signed PSBT returned by the external signer against the original one before accepting it. Without these checks, it is not super friendly (e.g. using RPC) to manually verify the same.

Note: the added tests were partially vibe-coded.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35358.

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Reviews

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LLM Linter (✨ experimental)

Possible places where named args for integral literals may be used (e.g. func(x, /*named_arg=*/0) in C++, and func(x, named_arg=0) in Python):

• walletcreatefundedpsbt([], {dest: 0.5}, 0, {...}, True) in test/functional/wallet_signer.py

^{2026-05-22 13:11:37}