Calling `initRustCrypto` Causes TypeError

Nobody has claimed this yet.

I'm writing an HTML page and want to enable e2ee. I've read and re-read the docs and some other implementations and I cannot figure out what I'm doing wrong here.

I've defined the following function to set up a matrix client, and initialize the rust crypto module. I haven't included either the `store` or `cryptoStore` parameters, as I don't have any legacy data to migrate.
```ts
export const initClient = async (session: Session): Promise => {
console.log("Initializing matrix", session);

const client = createClient({
baseUrl: session.baseUrl,
accessToken: session.accessToken,
userId: session.userId,
deviceId: session.deviceId,
timelineSupport: true,
useAuthorizationHeader: true,
verificationMethods: ["m.sas.v1"],
});

await client.initRustCrypto(); // Throws type error, despite the lack of legacy data

client.setMaxListeners(50);

return client;
};
```

When `initRustCrypto` is called, it throws the following TypeError:
```
TypeError: Cannot read properties of undefined (reading 'getMigrationState')
at _IndexedDBCryptoStore.getMigrationState
```

It appears to be being thrown here, which is SUPER odd.
```ts
// node_modules/matrix-js-sdk/src/rust-crypto/libolm_migration.ts
getMigrationState() {
return this.backend.getMigrationState();
}
```

Final thing that's happening is this **only happens on a page reload**, after successfully signing in and caching the session information. I've verified that I'm passing in the correct userId/deviceId/accessToken on the second sign in attempt.

Even if I pass a legacy crypto store it still hits the error!

```ts
const cryptoStore = new IndexedDBCryptoStore(window.indexedDB, "crypto-store");

const client = createClient({
baseUrl: session.baseUrl,
accessToken: session.accessToken,
userId: session.userId,
deviceId: session.deviceId,
timelineSupport: true,
cryptoStore: cryptoStore,
useAuthorizationHeader: true,
verificationMethods: ["m.sas.v1"],
});

await client.initRustCrypto(); // Still throws
```

Start with client.initRustCrypto and libolm_migration.ts, especially IndexedDBCryptoStore.getMigrationState, then reproduce the reported flow across a page reload with and without cryptoStore. Confirm the failure and trace which initialization state is missing; done means the reload path completes without the TypeError and has regression coverage for both configurations.

Written by the indexing model from the issue text.