on setProvider call, shutting down the previous provider should happen before creating the new one

Hi,

I noticed that if I use `setProvider` to set a provider, then later I want to replace it with another provider (in my case, I do this to supports changing the api key I use to connect to the provider without stopping my application, so I recreate the provider with the new key) the following happens in parallel:
- the shutting down of the old provider
- the initialization of the new one

For Unleash, this can be a problem as shown in those logs:
```
server-1 | {"timestamp":"2026-04-28T10:55:47.302964637Z","logger":"io.getunleash.repository.FeatureBackupHandlerFile","level":"INFO","thread":"openfeature-provider-thread-1","message":"Unleash will try to load feature toggle states from temporary backup"}
server-1 | {"timestamp":"2026-04-28T10:55:47.304799269Z","logger":"io.getunleash.repository.PollingFeatureFetcher","level":"INFO","thread":"openfeature-provider-thread-1","message":"Starting polling client"}
server-1 | {"timestamp":"2026-04-28T10:55:47.485565437Z","logger":"io.getunleash.DefaultUnleash","level":"ERROR","thread":"openfeature-provider-thread-1","message":"You already have 1 clients for AppName [my-app] with instanceId: [xxxxxxxx] running. Please double check your code where you are instantiating the Unleash SDK"}
server-1 | {"timestamp":"2026-04-28T10:55:47.485655434Z","logger":"dev.openfeature.contrib.providers.unleash.UnleashProvider","level":"INFO","thread":"openfeature-provider-thread-1","message":"finished initializing provider"}
server-1 | {"timestamp":"2026-04-28T10:55:47.485682289Z","logger":"dev.openfeature.sdk.FeatureProviderStateManager","level":"INFO","thread":"openfeature-provider-thread-1","message":"Provider Unleash transitioned from state NOT_READY to state READY"}
server-1 | {"timestamp":"2026-04-28T10:55:47.486912037Z","logger":"dev.openfeature.contrib.providers.unleash.UnleashProvider","level":"INFO","thread":"openfeature-provider-thread-2","message":"shutdown"}
server-1 |
```

As we can see the shutdown of the original client happens after the initialization of the new one and we get an ERROR message from the Unleash library.

Comienza en el punto de entrada setProvider y sigue cómo se detiene el proveedor existente y cómo se inicializa el reemplazo. Reproduce el reemplazo del proveedor mostrado en el issue y verifica después que la detención finaliza antes de que comience la inicialización y que el Unleash duplicate-client error ya no se produce.

Escrito por el modelo de indexación a partir del texto del issue.