### 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_
/** A helper function to construct the scripts for this descriptor.
     *
     *  This function is invoked once by ExpandHelper.
     *
     *  @param pubkeys The evaluations of the m_pubkey_args field.
     *  @param scripts The evaluations of m_subdescriptor_args (one for each m_subdescriptor_args element).
     *  @param out A FlatSigningProvider to put scripts or public keys in that are necessary to the solver.
     *             The origin info of the provided pubkeys is automatically added.
     *  @return A vector with scriptPubKeys for this descriptor.
     */
    virtual std::vector<CScript> MakeScripts(const std::vector<CPubKey>& pubkeys, Span<const CScript> scripts, FlatSigningProvider& out) const = 0;
public:
    DescriptorImpl(std::vector<std::unique_ptr<PubkeyProvider>> pubkeys, const std::string& name) : m_pubkey_args(std::move(pubkeys)), m_name(name), m_subdescriptor_args() {}
    DescriptorImpl(std::vector<std::unique_ptr<PubkeyProvider>> pubkeys, std::unique_ptr<DescriptorImpl> script, const std::string& name) : m_pubkey_args(std::move(pubkeys)), m_name(name), m_subdescriptor_args(Vector(std::move(script))) {}
    DescriptorImpl(std::vector<std::unique_ptr<PubkeyProvider>> pubkeys, std::vector<std::unique_ptr<DescriptorImpl>> scripts, const std::string& name) : m_pubkey_args(std::move(pubkeys)), m_name(name), m_subdescriptor_args(std::move(scripts)) {}
    enum class StringType
    {
        PUBLIC,
        PRIVATE,
        NORMALIZED,
    };
    bool IsSolvable() const override