fuzz: cover the mempool interface for transaction announcement #35856

 122 | +{
 123 | +    LOCK(tx_pool.cs);
 124 | +    std::vector<Wtxid> res;
 125 | +
 126 | +    uint8_t dummy{0};
 127 | +    const auto mempool_entries{tx_pool.entryAll()};

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 193 | +        // not correspond to a mempool entry..
 194 | +        const auto is_in_mempool = [&](const auto& wtxid) EXCLUSIVE_LOCKS_REQUIRED(tx_pool.cs) { return tx_pool.GetIter(wtxid).has_value(); };
 195 | +        Assert(WITH_LOCK(tx_pool.cs, return std::ranges::all_of(wtxids_to_relay, is_in_mempool)));
 196 | +        // ..As well as all duplicates.
 197 | +        const auto wtxids_count{wtxids_to_relay.size()};
 198 | +        const std::set<Wtxid> unique_wtxids{std::make_move_iterator(wtxids_to_relay.begin()), std::make_move_iterator(wtxids_to_relay.end())};

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