Fix: value too long error for chat name (#7697)

### 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):

api/apps/conversation_app.py

@@ -41,6 +41,11 @@ def set_conversation():
     req = request.json
     conv_id = req.get("conversation_id")
     is_new = req.get("is_new")
+    name = req.get("name", "New conversation")
+
+    if len(name) > 255:
+        name = name[0:255]
+
     del req["is_new"]
     if not is_new:
         del req["conversation_id"]
@@ -59,7 +64,7 @@ def set_conversation():
         e, dia = DialogService.get_by_id(req["dialog_id"])
         if not e:
             return get_data_error_result(message="Dialog not found")
-        conv = {"id": conv_id, "dialog_id": req["dialog_id"], "name": req.get("name", "New conversation"), "message": [{"role": "assistant", "content": dia.prompt_config["prologue"]}]}
+        conv = {"id": conv_id, "dialog_id": req["dialog_id"], "name": name, "message": [{"role": "assistant", "content": dia.prompt_config["prologue"]}]}
         ConversationService.save(**conv)
         return get_json_result(data=conv)
     except Exception as e: