### What problem does this PR solve?

Hello, when I input a very long line in the chat input box, it will fail
with following error:

```
2025-05-17 16:11:26,004 ERROR    182558 value too long for type character varying(255)
Traceback (most recent call last):
  File "/var/home/sfc/Projects/ragflow/.venv/lib/python3.10/site-packages/peewee.py", line 3291, in execute_sql
    cursor.execute(sql, params or ())
psycopg2.errors.StringDataRightTruncation: value too long for type character varying(255)


During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/var/home/sfc/Projects/ragflow/api/apps/conversation_app.py", line 68, in set_conversation
    ConversationService.save(**conv)
  File "/var/home/sfc/Projects/ragflow/.venv/lib/python3.10/site-packages/peewee.py", line 3128, in inner
    return fn(*args, **kwargs)
  File "/var/home/sfc/Projects/ragflow/api/db/services/common_service.py", line 145, in save
    return cls.save_n(**kwargs)
  File "/var/home/sfc/Projects/ragflow/api/db/services/common_service.py", line 139, in save_n
    sample_obj = cls.model(**kwargs).save(force_insert=True)
  File "/var/home/sfc/Projects/ragflow/.venv/lib/python3.10/site-packages/peewee.py", line 6923, in save
    pk = self.insert(**field_dict).execute()
  File "/var/home/sfc/Projects/ragflow/.venv/lib/python3.10/site-packages/peewee.py", line 2011, in inner
    return method(self, database, *args, **kwargs)
  File "/var/home/sfc/Projects/ragflow/.venv/lib/python3.10/site-packages/peewee.py", line 2082, in execute
    return self._execute(database)
  File "/var/home/sfc/Projects/ragflow/.venv/lib/python3.10/site-packages/peewee.py", line 2887, in _execute
    return super(Insert, self)._execute(database)
  File "/var/home/sfc/Projects/ragflow/.venv/lib/python3.10/site-packages/peewee.py", line 2598, in _execute
    cursor = self.execute_returning(database)
  File "/var/home/sfc/Projects/ragflow/.venv/lib/python3.10/site-packages/peewee.py", line 2605, in execute_returning
    cursor = database.execute(self)
  File "/var/home/sfc/Projects/ragflow/.venv/lib/python3.10/site-packages/peewee.py", line 3299, in execute
    return self.execute_sql(sql, params)
  File "/var/home/sfc/Projects/ragflow/.venv/lib/python3.10/site-packages/peewee.py", line 3289, in execute_sql
    with __exception_wrapper__:
  File "/var/home/sfc/Projects/ragflow/.venv/lib/python3.10/site-packages/peewee.py", line 3059, in __exit__
    reraise(new_type, new_type(exc_value, *exc_args), traceback)
  File "/var/home/sfc/Projects/ragflow/.venv/lib/python3.10/site-packages/peewee.py", line 192, in reraise
    raise value.with_traceback(tb)
  File "/var/home/sfc/Projects/ragflow/.venv/lib/python3.10/site-packages/peewee.py", line 3291, in execute_sql
    cursor.execute(sql, params or ())
peewee.DataError: value too long for type character varying(255)
```

This PR fix it by truncate the `name` field in the `set_conversation`
method in the `conversation_app.py`.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):