fix: 修复集群不可达时误报100分的严重bug

- 新增 check_connectivity() 连通性预检: kubectl存在性/kubeconfig/API Server连接
- 连不上集群直接返回0分+明确错误信息+排查建议(不再静默跳过)
- 每个检查项kubectl失败时必须报critical问题(不再忽略error)
- 前端: 连接失败时显示红色错误页+错误详情+排查建议+重新诊断按钮
- 智能识别常见错误: connection refused/证书过期/超时/DNS解析失败等

backend/main.py

@@ -13,6 +13,7 @@ from typing import Optional
 from backend.config import PORT
 from backend.diagnosis import (
     run_full_diagnosis, build_diagnosis_report, ALL_CHECKS,
+    check_connectivity,
     check_cluster_info, check_pods, check_nodes, check_events,
     check_deployments, check_services, check_pvc, check_resource_usage,
     check_network_policies,
@@ -82,12 +83,46 @@ async def diagnose_stream(namespace: str = ""):
 
     async def event_generator():
         start = datetime.now()
+
+        # ★ 第一步: 连通性检查
+        yield f"data: {json.dumps({'type': 'progress', 'step': 0, 'total': len(CHECK_ORDER) + 1, 'name': 'connectivity', 'title': '集群连通性检查', 'status': 'running'}, ensure_ascii=False)}\n\n"
+
+        conn = await check_connectivity()
+        if not conn["connected"]:
+            yield f"data: {json.dumps({'type': 'progress', 'step': 0, 'total': len(CHECK_ORDER) + 1, 'name': 'connectivity', 'title': '集群连通性检查', 'status': 'done', 'issues': 1}, ensure_ascii=False)}\n\n"
+
+            diagnosis = {
+                "timestamp": start.isoformat(),
+                "elapsed_seconds": round((datetime.now() - start).total_seconds(), 2),
+                "namespace": namespace or "all",
+                "score": 0,
+                "status": "critical",
+                "connected": False,
+                "connectivity_error": conn["error"],
+                "connectivity_suggestion": conn.get("suggestion", ""),
+                "critical_count": 1,
+                "warning_count": 0,
+                "issues": [{
+                    "level": "critical",
+                    "check": "集群连通性",
+                    "resource": "API Server",
+                    "message": f"无法连接到集群: {conn['error'][:300]}",
+                }],
+                "checks": {},
+            }
+            _last_diagnosis = diagnosis
+            _last_report = build_diagnosis_report(diagnosis)
+            yield f"data: {json.dumps({'type': 'complete', 'diagnosis': diagnosis}, ensure_ascii=False)}\n\n"
+            return
+
+        yield f"data: {json.dumps({'type': 'progress', 'step': 0, 'total': len(CHECK_ORDER) + 1, 'name': 'connectivity', 'title': '集群连通性检查', 'status': 'done', 'issues': 0}, ensure_ascii=False)}\n\n"
+
+        # ★ 第二步: 执行各项检查
         results = {}
         all_issues = []
-        total = len(CHECK_ORDER)
+        total = len(CHECK_ORDER) + 1
 
         for idx, (name, title, fn) in enumerate(CHECK_ORDER, 1):
-            # 推送: 开始检查
             yield f"data: {json.dumps({'type': 'progress', 'step': idx, 'total': total, 'name': name, 'title': title, 'status': 'running'}, ensure_ascii=False)}\n\n"
 
             try:
@@ -101,10 +136,13 @@ async def diagnose_stream(namespace: str = ""):
                     issue["check"] = title
                     all_issues.append(issue)
             except Exception as e:
-                results[name] = {"name": name, "title": title, "error": str(e), "issues": []}
-                issue_count = -1
+                results[name] = {
+                    "name": name, "title": title, "error": str(e),
+                    "issues": [{"level": "critical", "resource": name, "message": f"检查执行异常: {str(e)}"}],
+                }
+                issue_count = 1
+                all_issues.append({"level": "critical", "check": title, "resource": name, "message": f"检查执行异常: {str(e)}"})
 
-            # 推送: 完成检查
             yield f"data: {json.dumps({'type': 'progress', 'step': idx, 'total': total, 'name': name, 'title': title, 'status': 'done', 'issues': issue_count}, ensure_ascii=False)}\n\n"
 
         # 汇总
@@ -120,6 +158,7 @@ async def diagnose_stream(namespace: str = ""):
             "namespace": namespace or "all",
             "score": score,
             "status": status,
+            "connected": True,
             "critical_count": len(critical),
             "warning_count": len(warning),
             "issues": all_issues,
@@ -129,7 +168,6 @@ async def diagnose_stream(namespace: str = ""):
         _last_diagnosis = diagnosis
         _last_report = build_diagnosis_report(diagnosis)
 
-        # 推送: 最终结果
         yield f"data: {json.dumps({'type': 'complete', 'diagnosis': diagnosis}, ensure_ascii=False)}\n\n"
 
     return StreamingResponse(