State sync: authenticate chain-tip header via `block_signature`

Splitting this out of #1887 [comment ](https://github.com/0xMiden/miden-client/pull/1887#discussion_r3395347821) so it doesn't block the PR further. From there:

> Since 0xMiden/node#2075, `SyncChainMmrResponse` carries `block_signature` - but it's dropped in the `ChainMmrInfo`, so this assumption is still fully open.
>
> The signature doesn't make the tip *canonical* (a malicious validator still can't be caught - that's the "eventually… canonical" part), but it does let the client authenticate that the returned header was signed by the validator, shrinking the trusted set from *any serving endpoint* to *the validator*.
>
> Might be worth carrying it through `ChainMmrInfo` and verifying before the tip leaf is added (anchor the validator key to the current trusted header), rather than leaving the assumption open - or tracking as a follow-up.

The issue references a change in the node repository (0xMiden/node#2075) where `SyncChainMmrResponse` gained a `block_signature`. Look at the `ChainMmrInfo` struct in the rust-sdk to see where this signature is currently dropped. The work involves carrying the signature through this struct and adding verification logic before a tip leaf is added, likely in state synchronization code. Understanding the validator key anchoring to a trusted header is required.

索引モデルが issue の本文から書いたものです。