ServerValues.resolveDeferredValue has incorrect cast

* Operating System version: Gentoo Linux
* Firebase SDK version: 9.3.0
* Library version: 9.3.0 (but the bug goes back to at least 8.1.0)
* Firebase Product: RTDB

As [ServerValue.increment](firebase-admin-java#961) is still not supported by this SDK 4 years after its introduction, I tried implementing it manually. This triggers a `ClassCastException` inside the SDK due to a bad assumption that the value of the `.sv` key is always a `String`.

#### Steps to reproduce:

1. Create the data structure for ServerValue.increment as described in documentation (such as the REST documentation):
```
{
".sv": {
"increment": delta
}
}
```

2. Call `setValue` using this data structure.

#### Relevant Code:
```
var node = databaseNode();
node.setValue(Map.of(".sv", Map.of("increment", delta)), callback);
```

#### Error:

```
2024-06-21T19:58:12,008Z [firebase-database-worker] ERROR c.g.firebase.database.core.RunLoop - Uncaught exception in Firebase Database runloop (9.3.0). Please report to firebase-database-client@google.com
java.lang.ClassCastException: class java.util.HashMap cannot be cast to class java.lang.String (java.util.HashMap and java.lang.String are in module java.base of loader 'bootstrap')
at com.google.firebase.database.core.ServerValues.resolveDeferredValue(ServerValues.java:44)
at com.google.firebase.database.core.ServerValues.resolveDeferredValueSnapshot(ServerValues.java:80)
at com.google.firebase.database.core.Repo.setValue(Repo.java:376)
at com.google.firebase.database.DatabaseReference$1.run(DatabaseReference.java:305)
at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base/java.lang.Thread.run(Thread.java:829)
```

Start at ServerValues.resolveDeferredValue in ServerValues.java:44 and trace its callers through ServerValues.resolveDeferredValueSnapshot and Repo.setValue. Reproduce the failure through DatabaseReference.setValue using the documented map-shaped .sv increment value. Done means this input no longer fails with the reported ClassCastException, with behavior checked against the issue's reproduction steps.

索引モデルが issue の本文から書いたものです。