fix(agents): initialize branch in BaseAgent for proper propagation

- Ensure BaseAgent.createInvocationContext initializes the branch if empty.
- Clean up ParallelAgent by removing unused setBranchForCurrentAgent method.
- Fix NoSuchElementException in ParallelAgentTest.

Author: glaforge

core/src/main/java/com/google/adk/agents/BaseAgent.java

@@ -285,10 +285,12 @@ private InvocationContext createInvocationContext(InvocationContext parentContex
     InvocationContext.Builder builder = parentContext.toBuilder();
     builder.agent(this);
     // Check for branch to be truthy (not None, not empty string),
-    parentContext
-        .branch()
-        .filter(s -> !s.isEmpty())
-        .ifPresent(branch -> builder.branch(branch + "." + name()));
+    String parentBranch = parentContext.branch().filter(s -> !s.isEmpty()).orElse(null);
+    if (parentBranch == null) {
+      builder.branch(name());
+    } else {
+      builder.branch(parentBranch + "." + name());
+    }
     return builder.build();
   }
 

core/src/main/java/com/google/adk/agents/ParallelAgent.java

@@ -15,9 +15,6 @@
  */
 package com.google.adk.agents;
 
-import static com.google.common.base.Strings.isNullOrEmpty;
-import static com.google.common.collect.ImmutableList.toImmutableList;
-
 import com.google.adk.agents.ConfigAgentUtils.ConfigurationException;
 import com.google.adk.events.Event;
 import io.reactivex.rxjava3.core.Flowable;
@@ -108,25 +105,6 @@ public static ParallelAgent fromConfig(ParallelAgentConfig config, String config
     return agent;
   }
 
-  /**
-   * Sets the branch for the current agent in the invocation context.
-   *
-   * <p>Appends the agent name to the current branch, or sets it if undefined.
-   *
-   * @param currentAgent Current agent.
-   * @param invocationContext Invocation context to update.
-   * @return A new invocation context with branch set.
-   */
-  private static InvocationContext setBranchForCurrentAgent(
-      BaseAgent currentAgent, InvocationContext invocationContext) {
-    String branch = invocationContext.branch().orElse(null);
-    if (isNullOrEmpty(branch)) {
-      return invocationContext.toBuilder().branch(currentAgent.name()).build();
-    } else {
-      return invocationContext.toBuilder().branch(branch + "." + currentAgent.name()).build();
-    }
-  }
-
   /**
    * Runs sub-agents in parallel and emits their events.
    *