feat: Add OceanBase Performance Monitoring and Health Check Integration (#12886)

## Description

This PR implements comprehensive OceanBase performance monitoring and
health check functionality as requested in issue #12772. The
implementation follows the existing ES/Infinity health check patterns
and provides detailed metrics for operations teams.

## Problem

Currently, RAGFlow lacks detailed health monitoring for OceanBase when
used as the document engine. Operations teams need visibility into:
- Connection status and latency
- Storage space usage
- Query throughput (QPS)
- Slow query statistics
- Connection pool utilization

## Solution

### 1. Enhanced OBConnection Class (`rag/utils/ob_conn.py`)

Added comprehensive performance monitoring methods:
- `get_performance_metrics()` - Main method returning all performance
metrics
- `_get_storage_info()` - Retrieves database storage usage
- `_get_connection_pool_stats()` - Gets connection pool statistics
- `_get_slow_query_count()` - Counts queries exceeding threshold
- `_estimate_qps()` - Estimates queries per second
- Enhanced `health()` method with connection status

### 2. Health Check Utilities (`api/utils/health_utils.py`)

Added two new functions following ES/Infinity patterns:
- `get_oceanbase_status()` - Returns OceanBase status with health and
performance metrics
- `check_oceanbase_health()` - Comprehensive health check with detailed
metrics

### 3. API Endpoint (`api/apps/system_app.py`)

Added new endpoint:
- `GET /v1/system/oceanbase/status` - Returns OceanBase health status
and performance metrics

### 4. Comprehensive Unit Tests
(`test/unit_test/utils/test_oceanbase_health.py`)

Added 340+ lines of unit tests covering:
- Health check success/failure scenarios
- Performance metrics retrieval
- Error handling and edge cases
- Connection pool statistics
- Storage information retrieval
- QPS estimation
- Slow query detection

## Metrics Provided

- **Connection Status**: connected/disconnected
- **Latency**: Query latency in milliseconds
- **Storage**: Used and total storage space
- **QPS**: Estimated queries per second
- **Slow Queries**: Count of queries exceeding threshold
- **Connection Pool**: Active connections, max connections, pool size

## Testing

- All unit tests pass
- Error handling tested for connection failures
- Edge cases covered (missing tables, connection errors)
- Follows existing code patterns and conventions

## Code Statistics

- **Total Lines Changed**: 665+ lines
- **New Code**: ~600 lines
- **Test Coverage**: 340+ lines of comprehensive tests
- **Files Modified**: 3
- **Files Created**: 1 (test file)

## Acceptance Criteria Met

✅ `/system/oceanbase/status` API returns OceanBase health status
✅ Monitoring metrics accurately reflect OceanBase running status
✅ Clear error messages when health checks fail
✅ Response time optimized (metrics cached where possible)
✅ Follows existing ES/Infinity health check patterns
✅ Comprehensive test coverage

## Related Files

- `rag/utils/ob_conn.py` - OceanBase connection class
- `api/utils/health_utils.py` - Health check utilities
- `api/apps/system_app.py` - System API endpoints
- `test/unit_test/utils/test_oceanbase_health.py` - Unit tests

Fixes #12772

---------

Co-authored-by: Daniel <daniel@example.com>

api/apps/system_app.py

@@ -35,7 +35,7 @@ from timeit import default_timer as timer
 
 from rag.utils.redis_conn import REDIS_CONN
 from quart import jsonify
-from api.utils.health_utils import run_health_checks
+from api.utils.health_utils import run_health_checks, get_oceanbase_status
 from common import settings
 
 
@@ -182,6 +182,42 @@ async def ping():
     return "pong", 200
 
 
+@manager.route("/oceanbase/status", methods=["GET"])  # noqa: F821
+@login_required
+def oceanbase_status():
+    """
+    Get OceanBase health status and performance metrics.
+    ---
+    tags:
+      - System
+    security:
+      - ApiKeyAuth: []
+    responses:
+      200:
+        description: OceanBase status retrieved successfully.
+        schema:
+          type: object
+          properties:
+            status:
+              type: string
+              description: Status (alive/timeout).
+            message:
+              type: object
+              description: Detailed status information including health and performance metrics.
+    """
+    try:
+        status_info = get_oceanbase_status()
+        return get_json_result(data=status_info)
+    except Exception as e:
+        return get_json_result(
+            data={
+                "status": "error",
+                "message": f"Failed to get OceanBase status: {str(e)}"
+            },
+            code=500
+        )
+
+
 @manager.route("/new_token", methods=["POST"])  # noqa: F821
 @login_required
 def new_token():