fix(validator): also reject path-style DoH in dns_fallback_server

A value like 'cloudflare-dns.com/dns-query' is the sing-box DoH
path format. The previous check only caught 'https://' URLs.
dns_server_value_valid() accepts it because it validates only the
host part — so 'cloudflare-dns.com' passed as a valid hostname.

Fallback servers are plain UDP only; any '/' in the value is the
DoH path separator and must be rejected.

Changes:
- validator.uc: add index(value, '/') >= 0 check before the
  dns_server_value_valid call
- config_validator_runtime.sh: add regression test for path-style
  DoH ('cloudflare-dns.com/dns-query') in dns_fallback_server

tests/config_validator_runtime.sh

@@ -273,6 +273,20 @@ cat >"$WORK_DIR/url-in-fallback-dns.json" <<'JSON'
 JSON
 assert_rejects "URL in dns_fallback_server" "$WORK_DIR/url-in-fallback-dns.json" "Fallback DNS server"
 
+cat >"$WORK_DIR/path-doh-in-fallback-dns.json" <<'JSON'
+{
+  "settings": {
+    ".name": "settings",
+    ".type": "settings",
+    "dns_server": [ "1.1.1.1" ],
+    "bootstrap_dns_server": [ "77.88.8.8" ],
+    "dns_fallback_server": [ "cloudflare-dns.com/dns-query" ]
+  },
+  "section": []
+}
+JSON
+assert_rejects "path-style DoH in dns_fallback_server" "$WORK_DIR/path-doh-in-fallback-dns.json" "Fallback DNS server"
+
 cat >"$WORK_DIR/bad-dns-rule-matchers.json" <<'JSON'
 {
   "settings": { ".name": "settings", ".type": "settings" },