[Bug]: 条件过滤表达式中出现字典格式的字符串时会报错

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Is there an existing issue for this?

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Describe the bug

pymilvus/pymilvus/orm/iterator.py 第 375 行代码
filtered_pk_str = f'{self._pk_field_name} > "{self._next_id}"'

当 self._next_id 是一个字典格式的字符串时，比如 self._next_id = '{"name":"张三","school":"北大"}'

调用 collection.query_iterator() 迭代器会报如下错误：

pymilvus.exceptions.MilvusException: <MilvusException: (code=1100, message=failed to create query plan: cannot parse expression: ids > "{"name":"张三","school":"北大"}", error: invalid expression: ids > "{"name":"\u5f20\u4e09","school":"\u5317\u5927"}": invalid parameter)>

建议使用 json.dumps()
filtered_pk_str = f'{self._pk_field_name} > {json.dumps(self._next_id, ensure_ascii=True)}'

Expected Behavior

No response

Steps/Code To Reproduce behavior

Environment details

- Hardware/Softward conditions (OS, CPU, GPU, Memory):
- Method of installation (Docker, or from source):
- Milvus version (v0.3.1, or v0.4.0):
- Milvus configuration (Settings you made in `server_config.yaml`):

Anything else?

No response