kernel: validate C API inputs #35340

Fixes #35339.

This hardens the experimental libbitcoinkernel C API so caller-provided invalid runtime inputs fail through documented return/status channels instead of assertions. Existing script verification status numeric values are preserved, and the new values are appended.

Summary

• Return nullptr for out-of-range transaction, block transaction, block spent-output, and transaction spent-output accessors.
• Add script verification statuses for unsupported flag bits and out-of-range input indexes.
• Reject malformed precomputed transaction data spent-output arrays at construction.
• Document script verification failure statuses and precomputed spent-output array requirements.
• Add focused kernel API tests for the new failure behavior, including nullable status output handling.

Testing

• git diff --check
• cmake -B build-kernel-validation -DBUILD_KERNEL_LIB=ON -DBUILD_KERNEL_TEST=ON -DBUILD_TESTS=OFF -DBUILD_DAEMON=OFF -DBUILD_CLI=OFF -DBUILD_BITCOIN_BIN=OFF -DBUILD_TX=OFF -DBUILD_UTIL=OFF -DENABLE_WALLET=OFF -DENABLE_IPC=OFF
• cmake --build build-kernel-validation --target test_kernel -j $(sysctl -n hw.ncpu)
• build-kernel-validation/bin/test_kernel

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35340.

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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):

• btck_precomputed_transaction_data_create(tx.get(), too_many_spent_outputs, 2) in src/test/kernel/test_kernel.cpp
• btck_precomputed_transaction_data_create(tx.get(), nullptr, 1) in src/test/kernel/test_kernel.cpp
• btck_precomputed_transaction_data_create(tx.get(), null_spent_outputs, 1) in src/test/kernel/test_kernel.cpp

^{2026-05-20 22:20:57}

This change seems to do the exact opposite that the changes in #35312 propose. You also seem to have used an LLM to assist you with most of your work. Since we have limited review resources, and LLMs are capable of generating endless work for reviewers, we ask authors to make an effort in demonstrating that they are capable of explaining their own changes and have a good understanding of their contribution. In this case, I'm going to close this PR and the accompanying issue, and ask you instead to provide review to #35312 if you believe that asserting on these inputs is an anti-pattern.

Dupe of #33943?