script: qa: Improve `Key::Fingerprint` type safety #35606

Extracted from pseudoramdom's work in #35436:

Instead of using c style arrays for key fingerprints, use std::array's whose length can always reasoned about at compile time and for most operations the compiler enforces the size being correct.

using KeyFingerprint = std::array<unsigned char, 4>;

-    unsigned char vchFingerprint[4];
+    KeyFingerprint fingerprint;

This allows the replacement of a lot of raw memcpy + trust-me-bro lengths, with the assignment operator:

-    memcpy(ret.vchFingerprint, vchFingerprint, 4);
+    ret.fingerprint = fingerprint;

This commit also adds two helper functions for

• Retrieving the fingerprint of a key identifier (CKeyID)
• Retrieving the fingerprint of the key identifier of an XPUB.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35606.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #34697 (descriptor: fix musig duplicate checks and origin handling by shuv-amp)

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Concept ACK

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🚧 At least one of the CI tasks failed. _{Task iwyu: https://github.com/bitcoin/bitcoin/actions/runs/28208162547/job/83563390457} _{LLM reason (✨ experimental): CI failed because IWYU reported a header/include change needed in src/script/sign.cpp and was configured to fail when it made edits.}

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Instead of using vectors for key fingerprints, use std::array's whose length can be reasoned about at compile time.

-    unsigned char vchFingerprint[4];
+    KeyFingerprint fingerprint;

Just pointing out that before was not a std::vector, but a C-style array. It already has a fixed length that is enforced at compile time via sizeof.

Concept ACK. Thanks for taking care of it.

Just pointing out that before was not a std::vector, but a C-style array.

Oops, thanks for catching, fixed.

It already has a fixed length that is enforced at compile time via sizeof.

Yeah, the description was unclearly written

In some cases the length of a c-array is known at compile time, but since it decays to a pointer when passed as an argument, memory copying with a length argument has to be used.

The memcpy() caller could use sizeof() to get a length and pass this as an argument, but as can be seen in the existing code, that's often too inconvenient for people to do in practice, and unlike the assignment operator for a std::array size-correctness is not enforced by the compiler when using memcpy().