refactor: 清理代码并优化测试结构

1. 清理代码：
   - Fork.java: 移除注释和日志，保留简洁的 null 检查
   - Tip.java: 移除注释，保留防御性 null 检查

2. 优化测试结构：
   - shouldOkWhenAiFlowWithExampleSelector: 恢复为单次测试
   - shouldStableWhenRunnableParallelUnderConcurrency: 新增专门的并发稳定性测试（1000次重复）

详细的问题分析和修复说明已同步到 Issue #247

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: CodeCasterX

framework/fel/java/fel-core/src/main/java/modelengine/fel/core/util/Tip.java

@@ -118,7 +118,6 @@ public Tip addAll(Map<String, Content> args) {
      * @return 表示当前的 {@link Tip}。
      */
     public Tip merge(Tip other) {
-        // Issue #247: 防御性处理，在并发场景下 Fork.join() 可能传入 null
         if (other == null) {
             return this;
         }

framework/fel/java/fel-flow/src/test/java/modelengine/fel/engine/operators/PatternTest.java

@@ -81,8 +81,8 @@ void shouldOkWhenAiFlowWithNormalRunnableParallel() {
         assertThat(answer1.toString()).isEqualTo("answer question1 from context with my history");
     }
 
-    @RepeatedTest(1000)
-    @DisplayName("测试 ExampleSelector - 重复运行以复现 NPE")
+    @Test
+    @DisplayName("测试 ExampleSelector")
     void shouldOkWhenAiFlowWithExampleSelector() {
         Example[] examples = {new DefaultExample("2+2", "4"), new DefaultExample("2+3", "5")};
         Conversation<String, Prompt> converse = AiFlows.<String>create()
@@ -98,6 +98,23 @@ void shouldOkWhenAiFlowWithExampleSelector() {
         assertThat(converse.offer("1+2").await().text()).isEqualTo("2+2=4\n2+3=5\n1+2=");
     }
 
+    @RepeatedTest(1000)
+    @DisplayName("测试 RunnableParallel 并发稳定性")
+    void shouldStableWhenRunnableParallelUnderConcurrency() {
+        Example[] examples = {new DefaultExample("2+2", "4"), new DefaultExample("2+3", "5")};
+        Conversation<String, Prompt> converse = AiFlows.<String>create()
+                .runnableParallel(question(),
+                        fewShot(ExampleSelector.builder()
+                                .template("{{q}}={{a}}", "q", "a")
+                                .delimiter("\n")
+                                .example(examples)
+                                .build()))
+                .prompt(Prompts.human("{{examples}}\n{{question}}="))
+                .close()
+                .converse();
+        assertThat(converse.offer("1+2").await().text()).isEqualTo("2+2=4\n2+3=5\n1+2=");
+    }
+
     @Test
     @DisplayName("测试 Retriever")
     void shouldOkWhenAiFlowWithRetriever() {

framework/waterflow/java/waterflow-core/src/main/java/modelengine/fit/waterflow/domain/states/Fork.java

@@ -12,7 +12,6 @@
 import modelengine.fit.waterflow.domain.stream.reactive.Processor;
 import modelengine.fit.waterflow.domain.stream.reactive.Publisher;
 import modelengine.fit.waterflow.domain.utils.Tuple;
-import modelengine.fitframework.log.Logger;
 import modelengine.fitframework.util.ObjectUtils;
 
 import java.util.ArrayList;
@@ -34,8 +33,6 @@
  * @since 1.0
  */
 public class Fork<O, D, I, F extends Flow<D>> extends Activity<D, F> {
-    private static final Logger LOG = Logger.get(Fork.class);
-
     private final State<I, D, I, F> node;
 
     private final List<State<O, D, ?, F>> forks = new ArrayList<>();
@@ -97,21 +94,11 @@ public synchronized R process(FlowContext<O> input) {
                     }
                 }
 
-                // Issue #247: 智能处理并发场景下的 null 数据
-                // 在某些竞态条件下，FlowContext.data 可能为 null
-                // 此时不更新分支计数，等待正确的数据到来
                 O inputData = input.getData();
                 if (inputData == null) {
-                    LOG.warn("[Fork.join] Received null FlowContext.data, skipping. "
-                            + "key={}, session={}, thread={}, branch={}/{}, acc={}",
-                            key, input.getSession().getId(), Thread.currentThread().getName(),
-                            acc.second() + 1, forkNumber.get(), acc.first());
-                    // 返回 null 表示聚合未完成，等待有效数据
                     return null;
                 }
-
                 R processedResult = processor.process(acc.first(), inputData);
-
                 acc = Tuple.from(processedResult, acc.second() + 1);
                 accs.put(key, acc);