### 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_
{
    LOCK(cs_desc_man);
    return m_wallet_descriptor.creation_time;
}
std::unique_ptr<FlatSigningProvider> DescriptorScriptPubKeyMan::GetSigningProvider(const CScript& script, bool include_private) const
{
    LOCK(cs_desc_man);
    
    auto it = m_map_script_pub_keys.find(script);
    if (it == m_map_script_pub_keys.end()) {
        return nullptr;
    }
    int32_t index = it->second;
    return GetSigningProvider(index, include_private);
}
std::unique_ptr<FlatSigningProvider> DescriptorScriptPubKeyMan::GetSigningProvider(const CPubKey& pubkey) const
{
    LOCK(cs_desc_man);
    
    auto it = m_map_pubkeys.find(pubkey);
    if (it == m_map_pubkeys.end()) {
        return nullptr;
    }
    int32_t index = it->second;
    
    return GetSigningProvider(index, true);
}
std::unique_ptr<FlatSigningProvider> DescriptorScriptPubKeyMan::GetSigningProvider(int32_t index, bool include_private) const