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.

Written by the indexing model from the issue text.