interfaces: fix findAncestorByHash returning true for unknown descendant #36484

findAncestorByHash nulled out 'ancestor' only when the descendant block was also known, due to a 'block &&' guard. If block_hash didn't resolve to a known block, the guard short-circuited and the still-valid ancestor pointer was passed to FillBlock, causing the call to report success for an unknown descendant.

checked the sibling functions findCommonAncestor and findAncestorByHeight for the same short-circuit pattern; both already default to a null result whenever either lookup fails, so no equivalent bug exists there.

Extended the existing interfaces_tests/findAncestorByHash unit test with a regression assertion for an unknown descendant hash. Verified it fails without the fix and passes with it (./build/bin/test_bitcoin --run_test=interfaces_tests/findAncestorByHash).

fixes #36472

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36484
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36484.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36484#author-ralph

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Exact duplicate of #36473 (comment)