`PartialMmr` panics during `Client::sync_state()`(`miden-client`)

### Packages versions

miden-crypto 0.18.4 (pulled in via `miden-client` 0.12.3)

### Bug description

thread 'main' panicked at miden-crypto/src/merkle/mmr/partial.rs:396:9:
if there is an odd element, a merge is required

### How can this be reproduced?

1. My app invokes `Client::sync_state()` to refresh local state
2. The client has an existing partial MMR cache on disk.
3. Sync pulls the latest delta from a remote node and applies it via `PartialMmr::apply()`.
4. The sync path panics inside `miden-crypto`

This happens immediately after `Client::sync_state()` starts processing the returned MMR delta, so the downstream code never sees a recoverable error

### Relevant log output

```shell
thread 'main' panicked at miden-crypto/src/merkle/mmr/partial.rs:396:9:
if there is an odd element, a merge is required
```