Expose `ForeignAccount.prefetched()` and `getForeignAccountInputs()`

> One gap remains for us: the web SDK. `@miden-sdk/miden-sdk` 0.16.0 only exposes `ForeignAccount.public()` and `withForeignAccounts`, with no way to construct a prefetched account or to fetch `AccountInputs` from JS. Guardian proposals are exchanged between Rust and TS cosigners, so without a binding a TS client cannot use inputs a Rust proposer captured.

_Originally posted by @haseebrabbani in [#2524](https://github.com/0xMiden/rust-sdk/issues/2524#issuecomment-5631074129)_

Look for the ForeignAccount class in the web-sdk codebase, likely in a file like src/foreign-account.ts. Examine the existing public() and withForeignAccounts methods. The goal is to add a prefetched() method and a getForeignAccountInputs() function. Check how the Rust SDK handles these to understand the required data structures. Test by building the SDK and verifying the new methods are exported and usable from JavaScript.

Scritto dal modello di indicizzazione a partire dal testo della issue.