### 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_
{
    AssertLockHeld(cs_desc_man);
    std::unique_ptr<FlatSigningProvider> out_keys = std::make_unique<FlatSigningProvider>();
    
    auto it = m_map_signing_providers.find(index);
    if (it != m_map_signing_providers.end()) {
        out_keys->Merge(FlatSigningProvider{it->second});
    } else {
        
        std::vector<CScript> scripts_temp;
        if (!m_wallet_descriptor.descriptor->ExpandFromCache(index, m_wallet_descriptor.cache, scripts_temp, *out_keys)) return nullptr;
        
        m_map_signing_providers[index] = *out_keys;
    }
    if (HavePrivateKeys() && include_private) {
        FlatSigningProvider master_provider;
        master_provider.keys = GetKeys();
        m_wallet_descriptor.descriptor->ExpandPrivate(index, master_provider, *out_keys);
    }
    return out_keys;
}
std::unique_ptr<SigningProvider> DescriptorScriptPubKeyMan::GetSolvingProvider(const CScript& script) const
{
    return GetSigningProvider(script, false);
}
bool DescriptorScriptPubKeyMan::CanProvide(const CScript& script, SignatureData& sigdata)
{
    return IsMine(script);
}
bool DescriptorScriptPubKeyMan::SignTransaction(CMutableTransaction& tx, const std::map<COutPoint, Coin>& coins, int sighash, std::map<int, bilingual_str>& input_errors) const