fix: align aihr_practice_assignment.ext_party_id collation to utf8mb4_general_ci

根因:生产库 aihr_practice_assignment.ext_party_id 继承表级默认 utf8mb4_0900_ai_ci,
与 aihr_practice_question_feedback.ext_party_id (utf8mb4_general_ci) 冲突,
导致 AihrPracticeSeedService.assignments() 子查询抛 ERROR 1267,
/me 接口异常,员工端误报'人力系统未匹配'。

修复:
- ALTER TABLE 对齐 ext_party_id 列排序规则为 utf8mb4_general_ci
- 已在生产热修复并验证,查询恢复正常
- 本地 docker 容器复现+修复验证通过
- 前端无需改动,H5 重新构建确认流程无误

影响范围:
- 仅 aihr_practice_assignment 表单列 MODIFY
- 保留所有数据与索引,无业务中断
- 回滚材料在 /opt/wygj/backups/release-20260718005151

遗留:
- position 页异常场景退出入口待下轮补充(P1)
- schema 规范:后续 DDL 显式声明 COLLATE,不依赖表级默认

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/script/sql/update/aihr_20260718_hotfix_assignments_collation.sql

@@ -0,0 +1,32 @@
+-- Emergency collation alignment for aihr_practice_assignment.
+-- Root cause: production database uses utf8mb4_general_ci for tenant_id column
+-- (inherited from RuoYi base schema), but ext_party_id column lacks explicit
+-- COLLATE clause and inherits utf8mb4_0900_ai_ci from the table DEFAULT.
+-- When the subquery JOIN in AihrPracticeSeedService.assignments() compares
+-- aihr_practice_assignment.ext_party_id = aihr_practice_question_feedback.ext_party_id,
+-- MySQL rejects the implicit collation coercion with error 1267.
+--
+-- Fix: align ext_party_id column collation with the canonical tenant_id collation
+-- already in use by the target database (utf8mb4_general_ci in production).
+
+SET @assignments_target_collation := (
+  SELECT collation_name
+    FROM information_schema.columns
+   WHERE table_schema = DATABASE()
+     AND table_name = 'aihr_practice_assignment'
+     AND column_name = 'tenant_id'
+   LIMIT 1
+);
+SET @assignments_target_collation := COALESCE(@assignments_target_collation, 'utf8mb4_0900_ai_ci');
+
+SET @assignments_ddl := CONCAT(
+  'ALTER TABLE `aihr_practice_assignment` MODIFY COLUMN `ext_party_id` varchar(100) COLLATE ',
+  @assignments_target_collation,
+  ' NOT NULL COMMENT ''员工外部主体或手机号'''
+);
+PREPARE assignments_stmt FROM @assignments_ddl;
+EXECUTE assignments_stmt;
+DEALLOCATE PREPARE assignments_stmt;
+
+SET @assignments_ddl := NULL;
+SET @assignments_target_collation := NULL;