fix: align and correct eventBelongsToBranch to match _is_event_belongs_to_branch in adk-python (#437)

Author: sjy3

internal/llminternal/contents_processor.go

@@ -123,7 +123,7 @@ func buildContentsDefault(agentName, invocationBranch string, events []*session.
 }
 
 func eventBelongsToBranch(invocationBranch string, event *session.Event) bool {
-	if invocationBranch == "" {
+	if invocationBranch == "" || event.Branch == "" {
 		return true
 	}
 	if event.Branch == invocationBranch {

internal/llminternal/contents_processor_test.go

@@ -335,10 +335,18 @@ func TestContentsRequestProcessor(t *testing.T) {
 						Content: genai.NewContentFromText("In branch 2", "user"),
 					},
 				},
+				{
+					Author: "user",
+					Branch: "",
+					LLMResponse: model.LLMResponse{
+						Content: genai.NewContentFromText("empty branch", "user"),
+					},
+				},
 			},
 			want: []*genai.Content{
 				genai.NewContentFromText("In branch 1", "user"),
 				genai.NewContentFromText("In branch 1 and task 1", "user"),
+				genai.NewContentFromText("empty branch", "user"),
 			},
 		},
 		{