test: cover gettxspendingprevout error without txospenderindex #36573

Add functional coverage for the gettxspendingprevout error raised when mempool_only=false, no matching spend is in the mempool, and txospenderindex is unavailable.

The combined unit and functional coverage report shows this throw in src/rpc/mempool.cpp is never reached: https://github.com/bitcoin/bitcoin/blob/71fa0439278f1d11c9a1aabe67312dcdf1cc8757/src/rpc/mempool.cpp#L1080

The existing test already queries node2 without the index, but mempool_only defaults to true in that case, so the error is only reachable when mempool_only=false is passed explicitly.

The test now checks both sides on node2 with mempool_only=false: it succeeds while the spends are still in the mempool, and raises the error once they are mined.

<!--e57a25ab6845829454e8d69fc972939a-->

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

<!--006a51241073e994b41acfe9ec718e94-->

External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36573
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36573.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36573#author-ralph

<!--021abf342d371248e50ceaed478a90ca-->

Reviews

See the guideline and AI policy for information on the review process.

If your review is incorrectly listed, please copy-paste <code>&lt;!--meta-tag:bot-skip--&gt;</code> into the comment that the bot should ignore.

<!--5faf32d7da4f0f540f40219e4f7537a3-->

ACK edf0d13ad5ff9035ad7a0ff7bc38610edb952b2d

ACK https://github.com/bitcoin/bitcoin/commit/edf0d13ad5ff9035ad7a0ff7bc38610edb952b2d

ACK edf0d13ad5ff9035ad7a0ff7bc38610edb952b2d

@mercie-ux don't forget to remove the template text from your pull request description!