fix: 修复 AUTO 策略空分块兜底

- AUTO 推荐无分块时回退自然段长度拆分

- 保留默认或页面传入的分段长度与重叠参数

- 补充默认、自定义与显式策略回归测试

easyflow-modules/easyflow-module-ai/src/test/java/tech/easyflow/ai/documentimport/task/KnowledgeDocumentImportTaskAppServiceTest.java

@@ -7,6 +7,11 @@ import com.easyagents.core.model.embedding.EmbeddingModel;
 import com.easyagents.core.store.DocumentStore;
 import com.easyagents.core.store.StoreOptions;
 import com.easyagents.core.store.StoreResult;
+import com.easyagents.rag.core.RagChunk;
+import com.easyagents.rag.core.RagDefaults;
+import com.easyagents.rag.core.RagStrategyCodes;
+import com.easyagents.rag.ingestion.RagIngestionService;
+import com.easyagents.rag.ingestion.model.AnalysisResult;
 import com.easyagents.rag.ingestion.model.StrategyConfig;
 import com.easyagents.search.engine.service.DocumentSearcher;
 import org.apache.ibatis.annotations.Update;
@@ -1151,6 +1156,108 @@ public class KnowledgeDocumentImportTaskAppServiceTest {
         Mockito.verifyNoInteractions(searcher);
     }
 
+    /**
+     * 验证 AUTO 空结果使用默认长度与重叠参数回退自然段长度拆分。
+     *
+     * @throws Exception 反射调用异常
+     */
+    @Test
+    public void autoEmptyChunksShouldFallbackWithDefaultParagraphSettings()
+        throws Exception {
+        assertAutoParagraphFallback(RagDefaults.CHUNK_SIZE, RagDefaults.OVERLAP_SIZE);
+    }
+
+    /**
+     * 验证手动预览选择 AUTO 时，兜底保留页面传入的长度与重叠参数。
+     *
+     * @throws Exception 反射调用异常
+     */
+    @Test
+    public void autoEmptyChunksShouldFallbackWithRequestedParagraphSettings()
+        throws Exception {
+        assertAutoParagraphFallback(768, 192);
+    }
+
+    /**
+     * 验证显式选择非 AUTO 策略时不触发兜底。
+     *
+     * @throws Exception 反射调用异常
+     */
+    @Test
+    public void explicitStrategyEmptyChunksShouldNotFallback() throws Exception {
+        KnowledgeDocumentImportTaskAppService service =
+            new KnowledgeDocumentImportTaskAppService();
+        RagIngestionService ragIngestionService = Mockito.mock(RagIngestionService.class);
+        setField(service, "ragIngestionService", ragIngestionService);
+        AnalysisResult analysis = Mockito.mock(AnalysisResult.class);
+        StrategyConfig requestedStrategy = StrategyConfig.defaults();
+        requestedStrategy.setStrategyCode(RagStrategyCodes.OUTLINE_SECTION);
+        StrategyConfig effectiveStrategy = requestedStrategy.copy();
+        Mockito.when(ragIngestionService.split(analysis, effectiveStrategy))
+            .thenReturn(List.of());
+
+        List<RagChunk> chunks = invokeAutoParagraphFallback(
+            service, analysis, requestedStrategy, effectiveStrategy);
+
+        Assert.assertTrue(chunks.isEmpty());
+        Assert.assertEquals(RagStrategyCodes.OUTLINE_SECTION, effectiveStrategy.getStrategyCode());
+        Mockito.verify(ragIngestionService).split(analysis, effectiveStrategy);
+    }
+
+    private static void assertAutoParagraphFallback(int chunkSize, int overlapSize) throws Exception {
+        KnowledgeDocumentImportTaskAppService service =
+            new KnowledgeDocumentImportTaskAppService();
+        RagIngestionService ragIngestionService = Mockito.mock(RagIngestionService.class);
+        setField(service, "ragIngestionService", ragIngestionService);
+        AnalysisResult analysis = Mockito.mock(AnalysisResult.class);
+        StrategyConfig requestedStrategy = StrategyConfig.defaults();
+        requestedStrategy.setChunkSize(chunkSize);
+        requestedStrategy.setOverlapSize(overlapSize);
+        StrategyConfig effectiveStrategy = requestedStrategy.copy();
+        effectiveStrategy.setStrategyCode(RagStrategyCodes.OUTLINE_SECTION);
+        RagChunk fallbackChunk = new RagChunk();
+        List<String> invokedStrategies = new ArrayList<String>();
+
+        Mockito.when(ragIngestionService.split(
+                Mockito.eq(analysis), Mockito.any(StrategyConfig.class)))
+            .thenAnswer(invocation -> {
+                StrategyConfig config = invocation.getArgument(1);
+                invokedStrategies.add(
+                    config.getStrategyCode() + ":" + config.getChunkSize() + ":" + config.getOverlapSize());
+                return invokedStrategies.size() == 1
+                    ? List.of()
+                    : List.of(fallbackChunk);
+            });
+
+        List<RagChunk> chunks = invokeAutoParagraphFallback(
+            service, analysis, requestedStrategy, effectiveStrategy);
+
+        Assert.assertEquals(List.of(fallbackChunk), chunks);
+        Assert.assertEquals(
+            List.of(
+                RagStrategyCodes.OUTLINE_SECTION + ":" + chunkSize + ":" + overlapSize,
+                RagStrategyCodes.PARAGRAPH_LENGTH + ":" + chunkSize + ":" + overlapSize),
+            invokedStrategies);
+        Assert.assertEquals(
+            RagStrategyCodes.PARAGRAPH_LENGTH, effectiveStrategy.getStrategyCode());
+    }
+
+    @SuppressWarnings("unchecked")
+    private static List<RagChunk> invokeAutoParagraphFallback(
+        KnowledgeDocumentImportTaskAppService service,
+        AnalysisResult analysis,
+        StrategyConfig requestedStrategy,
+        StrategyConfig effectiveStrategy) throws Exception {
+        Method method = KnowledgeDocumentImportTaskAppService.class.getDeclaredMethod(
+            "splitWithAutoParagraphFallback",
+            AnalysisResult.class,
+            StrategyConfig.class,
+            StrategyConfig.class);
+        method.setAccessible(true);
+        return (List<RagChunk>) method.invoke(
+            service, analysis, requestedStrategy, effectiveStrategy);
+    }
+
     private static DocumentMapper mockDocumentMapper(tech.easyflow.ai.entity.Document persistedDocument,
                                                      AtomicReference<tech.easyflow.ai.entity.Document> updatedDocumentRef) {
         return (DocumentMapper) Proxy.newProxyInstance(