ServerValues.resolveDeferredValue has incorrect cast

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.

• 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 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)