fix: 修复 #591 POC 对 HTTPS 端口误用 HTTP + #592 空指针 panic

1. buildTargetURL 对 443/8443 等已知 TLS 端口默认使用 https scheme，
   webtitle 触发 POC 扫描前将检测到的协议写回 info.URL，
   避免对 HTTPS 服务发送 HTTP 请求导致 EOF
2. GetInfo 添加 probe nil 检查，防止探针初始化失败时空指针 panic
3. 删除 test-nuclei-example.yaml 测试模板，避免 robots.txt 误报

webscan/pocs/test-nuclei-example.yaml

@@ -1,26 +0,0 @@
-id: test-nuclei-example
-info:
-  name: Test Nuclei Example Template
-  author: fscan-dev
-  severity: info
-  description: |
-    This is a test template to demonstrate Nuclei format support in fscan.
-    It will be automatically converted to fscan format during loading.
-  reference:
-    - https://github.com/shadow1ng/fscan
-
-http:
-  - method: GET
-    path:
-      - "{{BaseURL}}/robots.txt"
-
-    matchers:
-      - type: word
-        words:
-          - "User-agent"
-          - "Disallow"
-        condition: and
-
-      - type: status
-        status:
-          - 200