rpc: flag invalid taproot leaf control blocks in decodepsbt #36382

decodepsbt shows each PSBT_IN_TAP_LEAF_SCRIPT record as-is. It does not say whether its control block could be a genuine leaf of the input's taproot tree. InferTaprootTree() already checks this, but silently skips whatever fails. That is correct there: one bad candidate among several script paths is not an error. But it means a PSBT with an unusable leaf shows no sign of it.

This PR adds a diagnostic only; signing does not change. A new optional control_block_warnings array, next to control_blocks, gives the first check a control block fails. There are four checks:

• leaf version (must be even);
• control block size;
• leaf-version byte (must match leaf_ver);
• Merkle root (must match PSBT_IN_TAP_MERKLE_ROOT).

The Merkle-root check needs PSBT_IN_TAP_MERKLE_ROOT, which is optional per BIP371. When it is absent, only the other three checks run, and the help text says so.

Similar in spirit to #25513, which added the same kind of check for PSBT_OUT_TAP_TREE.

Commits

1. Unit test for the four checks in InferTaprootTree() (none existed).
2. Extract the checks into CheckTapLeafCandidate(), shared by InferTaprootTree() and decodepsbt.
3. decodepsbt's new control_block_warnings field, with its functional test.

Written with LLM, a computer and the Internet; the mistakes, as usual, are all mine.

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36382
• Ralph review: https://git.fish.foo/bitcoin/bitcoin/pulls/36382#author-ralph

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Reviews

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #36224 (test: Add test coverage for PartiallySignedTransaction::Merge() by nebula-21)
• #36122 (BIP460: CISA for Taproot key path spends by fjahr)
• #35747 (wallet: Fix FillPSBT failing to sign owned inputs when UTXOs disagree by nervana21)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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🚧 At least one of the CI tasks failed. _{Task previous releases: https://github.com/bitcoin/bitcoin/actions/runs/36594607538/job/109496162271} _{LLM reason (✨ experimental): CI failed due to a C++ compilation error in src/test/psbt_tests.cpp (lambda uses leaf_ver illegally: “local variable ‘leaf_ver’ may not appear in this context”).}

<details><summary>Hints</summary>

Try to run the tests locally, according to the documentation. However, a CI failure may still happen due to a number of reasons, for example:

• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

Leave a comment here, if you need help tracking down a confusing failure.

</details>