Investigate Deadlock Prevention in `WpApiCache::execute()`

[oguzkocer]

The WpApiCache::execute() function can deadlock when nested calls are made through cloned cache references. While the docs note this is "unlikely to write accidentally," it's still possible in real scenarios like callback chains or cross-crate usage.

let cache2 = cache.clone();
cache.execute(|conn| {
    cache2.execute(|conn2| { ... })  // ⚠️ Deadlock!
})?;