feat: 新增编码预览功能

- 后端新增 /rpc-api/base/code-gen/preview 接口
- SequenceAllocator.preview 查询序号但不消耗
- 前端操作列新增预览按钮，弹窗展示下一个编码

nl-module-base/nl-module-base-api/src/main/java/cn/code/nl/module/base/api/codegen/CodeGenApi.java

@@ -9,6 +9,8 @@ import jakarta.validation.Valid;
 import org.springframework.cloud.openfeign.FeignClient;
 import org.springframework.web.bind.annotation.PostMapping;
 import org.springframework.web.bind.annotation.RequestBody;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RequestParam;
 
 /**
  * RPC 服务 - 编码生成
@@ -31,4 +33,8 @@ public interface CodeGenApi {
     @Operation(summary = "生成编码")
     CommonResult<String> generate(@Valid @RequestBody CodeGenerateReqDTO reqDTO);
 
+    @GetMapping(PREFIX + "/preview")
+    @Operation(summary = "预览下一个编码（不消耗序号）")
+    CommonResult<String> preview(@RequestParam("ruleCode") String ruleCode);
+
 }