[Bug]: savepoint error when using oceanbase as database

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### Dify version

1.15.0

### Cloud or Self Hosted

Self Hosted (Docker)

### Steps to reproduce

After deploying Dify with OceanBase as the backend database, inviting a new user consistently triggers a SAVEPOINT error in the application logs.

### ✔️ Expected Behavior

Dify should successfully invite a new user when using OceanBase as the backend database, without generating any SAVEPOINT-related errors.

### ❌ Actual Behavior

Register failed

```
Traceback (most recent call last):
File "sqlalchemy/engine/base.py", line 1967, in _exec_single_context
self.dialect.do_execute(
File "sqlalchemy/engine/default.py", line 952, in do_execute
cursor.execute(statement, parameters)
File "pymysql/cursors.py", line 153, in execute
result = self._query(query)
File "pymysql/cursors.py", line 322, in _query
conn.query(q)
File "pymysql/connections.py", line 575, in query
self._affected_rows = self._read_query_result(unbuffered=unbuffered)
File "pymysql/connections.py", line 826, in _read_query_result
result.read()
File "pymysql/connections.py", line 1203, in read
first_packet = self.connection._read_packet()
File "pymysql/connections.py", line 782, in _read_packet
packet.raise_for_error()
File "pymysql/protocol.py", line 219, in raise_for_error
err.raise_mysql_exception(self._data)
File "pymysql/err.py", line 150, in raise_mysql_exception
raise errorclass(errno, errval)
OperationalError: (1305, 'savepoint does not exist')

Traceback (most recent call last):
File "services/account_service.py", line 1948, in register
account = AccountService.create_account(
File "services/account_service.py", line 475, in create_account
session.commit()
File "sqlalchemy/orm/scoping.py", line 597, in commit
return self._proxied.commit()
File "sqlalchemy/orm/session.py", line 2030, in commit
trans.commit(_to_root=True)
File "", line 2, in commit
File "sqlalchemy/orm/state_changes.py", line 137, in _go
ret_value = fn(self, *arg, **kw)
File "sqlalchemy/orm/session.py", line 1318, in commit
trans.commit()
File "sqlalchemy/engine/base.py", line 2640, in commit
self._do_commit()
File "sqlalchemy/engine/base.py", line 2865, in _do_commit
self.connection._release_savepoint_impl(self._savepoint)
File "sqlalchemy/engine/base.py", line 1170, in _release_savepoint_impl
self.engine.dialect.do_release_savepoint(self, name)
File "sqlalchemy/engine/default.py", line 774, in do_release_savepoint
connection.execute(expression.ReleaseSavepointClause(name))
File "sqlalchemy/engine/base.py", line 1419, in execute
return meth(
File "sqlalchemy/sql/elements.py", line 527, in _execute_on_connection
return connection._execute_clauseelement(
File "sqlalchemy/engine/base.py", line 1641, in _execute_clauseelement
ret = self._execute_context(
File "sqlalchemy/engine/base.py", line 1846, in _execute_context
return self._exec_single_context(
File "sqlalchemy/engine/base.py", line 1986, in _exec_single_context
self._handle_dbapi_exception(
File "sqlalchemy/engine/base.py", line 2363, in _handle_dbapi_exception
raise sqlalchemy_exception.with_traceback(exc_info[2]) from e
File "sqlalchemy/engine/base.py", line 1967, in _exec_single_context
self.dialect.do_execute(
File "sqlalchemy/engine/default.py", line 952, in do_execute
cursor.execute(statement, parameters)
File "pymysql/cursors.py", line 153, in execute
result = self._query(query)
File "pymysql/cursors.py", line 322, in _query
conn.query(q)
File "pymysql/connections.py", line 575, in query
self._affected_rows = self._read_query_result(unbuffered=unbuffered)
File "pymysql/connections.py", line 826, in _read_query_result
result.read()
File "pymysql/connections.py", line 1203, in read
first_packet = self.connection._read_packet()
File "pymysql/connections.py", line 782, in _read_packet
packet.raise_for_error()
File "pymysql/protocol.py", line 219, in raise_for_error
err.raise_mysql_exception(self._data)
File "pymysql/err.py", line 150, in raise_mysql_exception
raise errorclass(errno, errval)
OperationalError: (pymysql.err.OperationalError) (1305, 'savepoint does not exist')
[SQL: RELEASE SAVEPOINT sa_savepoint_1]
(Background on this error at: https://sqlalche.me/e/20/e3q8)
```

Start by reproducing the user-registration failure with Dify 1.15.0 deployed using OceanBase, then trace the transaction and savepoint handling around `services/account_service.py`, especially `create_account` and `register`. The report includes a SQLAlchemy traceback ending in `RELEASE SAVEPOINT sa_savepoint_1`; use it to investigate where the savepoint is lost. Done means inviting a user with OceanBase succeeds without the SAVEPOINT error.

Written by the indexing model from the issue text.