Fetch the fee faucet id from the latest stored block header instead of the genesis

Originally discussed in [this comment](https://github.com/0xMiden/faucet/pull/262#discussion_r3769759885).

After https://github.com/0xMiden/rust-sdk/pull/2397 was merged, we should update the [`fetch_fee_faucet_id`](https://github.com/0xMiden/faucet/blob/5bc853758e0468176a9b9edb5997b5cb1d93e021/crates/faucet/src/lib.rs#L978) function so that it calls `Client::get_latest_block_header` instead of `Client::get_block_header_by_number`.

函式 `fetch_fee_faucet_id` 位於 crates/faucet/src/lib.rs 的第 978 行。查看目前對 `Client::get_block_header_by_number` 的呼叫，並將其替換為 `Client::get_latest_block_header`。查看 rust-sdk 的 PR #2397 以了解更改的背景。執行 faucet 測試以確保更新正常運作。

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