fix: 添加学生管理学校树接口

前端调用 /business/student/schoolTree 接口获取学校树结构，
但后端缺失此接口导致路径匹配到 /{studentId} 引发类型转换错误。

backend/ruoyi-modules/pangu-business/src/main/java/org/dromara/pangu/student/controller/PgStudentController.java

@@ -8,11 +8,16 @@ import org.dromara.common.log.enums.BusinessType;
 import org.dromara.common.mybatis.core.page.PageQuery;
 import org.dromara.common.mybatis.core.page.TableDataInfo;
 import org.dromara.common.web.core.BaseController;
+import org.dromara.pangu.school.domain.PgSchool;
+import org.dromara.pangu.school.domain.vo.SchoolTreeNode;
+import org.dromara.pangu.school.service.IPgSchoolService;
 import org.dromara.pangu.student.domain.PgStudent;
 import org.dromara.pangu.student.service.IPgStudentService;
 import org.springframework.validation.annotation.Validated;
 import org.springframework.web.bind.annotation.*;
 
+import java.util.List;
+
 /**
  * 学生管理
  *
@@ -25,6 +30,7 @@ import org.springframework.web.bind.annotation.*;
 public class PgStudentController extends BaseController {
 
     private final IPgStudentService studentService;
+    private final IPgSchoolService schoolService;
 
     @SaCheckPermission("business:student:list")
     @GetMapping("/list")
@@ -32,6 +38,14 @@ public class PgStudentController extends BaseController {
         return studentService.selectPageList(student, pageQuery);
     }
 
+    /**
+     * 获取学校树结构（用于学生管理左侧筛选）
+     */
+    @GetMapping("/schoolTree")
+    public R<List<SchoolTreeNode>> schoolTree() {
+        return R.ok(schoolService.selectSchoolTree(new PgSchool()));
+    }
+
     @SaCheckPermission("business:student:query")
     @GetMapping("/{studentId}")
     public R<PgStudent> getInfo(@PathVariable Long studentId) {