fix: true parallel tool execution via subscribeOn(Schedulers.io()) in concatMapEager

Fixes #1152

Author: YuqiGuo105

core/src/main/java/com/google/adk/flows/llmflows/Functions.java

@@ -47,6 +47,7 @@
 import io.reactivex.rxjava3.core.Single;
 import io.reactivex.rxjava3.disposables.Disposable;
 import io.reactivex.rxjava3.functions.Function;
+import io.reactivex.rxjava3.schedulers.Schedulers;
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.HashSet;
@@ -160,7 +161,9 @@ public static Maybe<Event> handleFunctionCalls(
     } else {
       functionResponseEventsObservable =
           Observable.fromIterable(validFunctionCalls)
-              .concatMapEager(call -> functionCallMapper.apply(call).toObservable());
+              .concatMapEager(
+                  call ->
+                      functionCallMapper.apply(call).toObservable().subscribeOn(Schedulers.io()));
     }
     return functionResponseEventsObservable
         .toList()
@@ -231,7 +234,9 @@ public static Maybe<Event> handleFunctionCallsLive(
     } else {
       responseEventsObservable =
           Observable.fromIterable(validFunctionCalls)
-              .concatMapEager(call -> functionCallMapper.apply(call).toObservable());
+              .concatMapEager(
+                  call ->
+                      functionCallMapper.apply(call).toObservable().subscribeOn(Schedulers.io()));
     }
 
     return responseEventsObservable