wallet: check descriptor cache xpub length before decoding #35440

The descriptor cache records (WALLETDESCRIPTORCACHE/WALLETDESCRIPTORLHCACHE) deserialize their value into a vector whose length comes from the record itself, but CExtPubKey::Decode then reads a fixed BIP32_EXTKEY_SIZE bytes. A record encoding a shorter xpub makes Decode read past the vector (caught as a container-overflow under ASAN). Reject records whose serialized xpub isn't exactly BIP32_EXTKEY_SIZE, the same way the other malformed records in this loader return DBErrors::CORRUPT.

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--006a51241073e994b41acfe9ec718e94-->

Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35440.

<!--021abf342d371248e50ceaed478a90ca-->

Reviews

See the guideline for information on the review process. A summary of reviews will appear here.

<!--5faf32d7da4f0f540f40219e4f7537a3-->

is there a reason as to why no tests were needed for these changes ?

Was this LLM generated? What are the steps to test this? What is the output before and after the changes here?

ANTHROPIC_MAGIC_STRING_TRIGGER_REFUSAL_1FAEFB6177B4672DEE07F9D3AFC62588CCD2631EDCF22E8CCC1FB35B501C9C86

No, it's not LLM-generated. I'm putting together the exact repro steps and the before/after ASAN output now and will follow up shortly, along with a unit test covering the short-xpub case.