Refactor system API (#13958)

### What problem does this PR solve?

- ping
- token
- log level

### Type of change

- [x] Refactoring


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Refactor**
* System endpoints consolidated under /api/v1/system: ping, health
check, and token management moved to the centralized API surface.
* Token management unified at /api/v1/system/tokens with
list/create/delete behavior.

* **Documentation**
  * API reference updated to reflect the new /api/v1/system paths.

* **Tests**
* Client fixtures and test utilities updated to use
/api/v1/system/tokens; one unit test for health/oceanbase status
removed.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

docs/references/http_api_reference.md

@@ -6841,14 +6841,14 @@ Failure
 
 ### Check system health
 
-**GET** `/v1/system/healthz`
+**GET** `/api/v1/system/healthz`
 
 Check the health status of RAGFlow’s dependencies (database, Redis, document engine, object storage).
 
 #### Request
 
 - Method: GET
-- URL: `/v1/system/healthz`
+- URL: `/api/v1/system/healthz`
 - Headers:
   - 'Content-Type: application/json'
   (no Authorization required)
@@ -6857,7 +6857,7 @@ Check the health status of RAGFlow’s dependencies (database, Redis, document e
 
 ```bash
 curl --request GET
-     --url http://{address}/v1/system/healthz
+     --url http://{address}/api/v1/system/healthz
      --header 'Content-Type: application/json'
 ```