feature:Add OceanBase Support to Text-to-SQL Agent (#12919)

### What problem does this PR solve?

Close #12768.

This PR adds OceanBase support to RAGFlow’s Text-to-SQL (ExeSQL)
component.
OceanBase is integrated via MySQL compatibility mode, and the UI
`db_type` options are updated accordingly.

### Type of change

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

### Changes

**Backend**
- Add `oceanbase` `db_type` validation and connection logic in
`exesql.py` and reuse existing MySQL compatibility mode

**Frontend**
- Add OceanBase option to the ExeSQL `db_type` selector

### How to test
1. Configure OceanBase connection in ExeSQL node
(host/port/user/password/database)
2. Input: “Show 10 rows from test table”
3. Generated SQL: `SELECT * FROM test LIMIT 10;`
4. Query executes successfully and results are returned

### Screenshots
- ExeSQL db_type includes OceanBase
<img width="649" height="1015" alt="2"
src="https://github.com/user-attachments/assets/e0a5f7b9-e282-402a-8639-64c1aef8fce6"
/>

- ExeSQL test OceanBase connection
<img width="2247" height="1140" alt="test_ob"
src="https://github.com/user-attachments/assets/f16ebd93-b48e-4d18-b53f-8496581e755d"
/>



- Query results from OceanBase shown in UI
<img width="2550" height="1351" alt="1"
src="https://github.com/user-attachments/assets/b44163dc-baab-420d-b31e-b644bdcb77a9"
/>

api/apps/canvas_app.py

@@ -326,6 +326,9 @@ async def test_db_connect():
         if req["db_type"] in ["mysql", "mariadb"]:
             db = MySQLDatabase(req["database"], user=req["username"], host=req["host"], port=req["port"],
                                password=req["password"])
+        elif req["db_type"] == "oceanbase":
+            db = MySQLDatabase(req["database"], user=req["username"], host=req["host"], port=req["port"],
+                               password=req["password"], charset="utf8mb4")
         elif req["db_type"] == 'postgres':
             db = PostgresqlDatabase(req["database"], user=req["username"], host=req["host"], port=req["port"],
                                     password=req["password"])