### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current behaviour

I randomly encountered this by loading a wallet created from master with an older version. I could trace it down to bd13dc2f46ea10302a928fcf0f53b7aed77ad260 (#26076), but I don't know descriptor wallets enough to understand what's going on (cc @Sjors @achow101).

### Expected behaviour

`bitcoind` starts, or at least tells me to use a later version if #26076 was a breaking change instead of corrupting `wallet.dat`.

### Steps to reproduce

(master, clean chain): 
`createwallet -regtest ""`
(v25.0rc2):
first `bitcoind -regtest` succeeds
then stop and restart the node: The second `bitcoind -regtest` leads to an InitError:
`Error: Error: Unable to expand wallet descriptor from cache`
now, the wallet cannot be loaded by either version.

### Relevant log output

_No response_

### How did you obtain Bitcoin Core

Compiled from source

### What version of Bitcoin Core are you using?

v25.0rc2 / master

### Operating system and version

Ubuntu

### Machine specifications

_No response_
{
    const auto& it = m_last_hardened_xpubs.find(key_exp_pos);
    if (it == m_last_hardened_xpubs.end()) return false;
    xpub = it->second;
    return true;
}
DescriptorCache DescriptorCache::MergeAndDiff(const DescriptorCache& other)
{
    DescriptorCache diff;
    for (const auto& parent_xpub_pair : other.GetCachedParentExtPubKeys()) {
        CExtPubKey xpub;
        if (GetCachedParentExtPubKey(parent_xpub_pair.first, xpub)) {
            if (xpub != parent_xpub_pair.second) {
                throw std::runtime_error(std::string(__func__) + ": New cached parent xpub does not match already cached parent xpub");
            }
            continue;
        }
        CacheParentExtPubKey(parent_xpub_pair.first, parent_xpub_pair.second);
        diff.CacheParentExtPubKey(parent_xpub_pair.first, parent_xpub_pair.second);
    }
    for (const auto& derived_xpub_map_pair : other.GetCachedDerivedExtPubKeys()) {
        for (const auto& derived_xpub_pair : derived_xpub_map_pair.second) {
            CExtPubKey xpub;
            if (GetCachedDerivedExtPubKey(derived_xpub_map_pair.first, derived_xpub_pair.first, xpub)) {
                if (xpub != derived_xpub_pair.second) {
                    throw std::runtime_error(std::string(__func__) + ": New cached derived xpub does not match already cached derived xpub");
                }
                continue;
            }