test: check algorithm after SelectionResult::Merge #36614

The wallet merges preset (MANUAL) inputs with the automatic coin selection result in spend.cpp. With the mutant applied, the merged result is incorrectly reported as MANUAL instead of the algorithm that actually selected the coins.

The test checks both cases:

• Preset inputs merged into an automatic result should keep the automatic algorithm.
• A MANUAL result merged with another result should take the algorithm of the other result.

With the mutant applied, both checks fail with manual != bnb.

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36614
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36614.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36614#author-ralph

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cc @murchandamus

ACK 4959d34130

This covers the surviving mutant https://bitcoincore.space/src/wallet/coinselection.cpp#3707. I ran bcore-mutation and confirmed that merge_algo_test kills it.