fix: bracket a bare IPv6 ID server when deriving the API server (#16624)

With the API server field empty, get_api_server_ builds the URL from the
ID server as "http://{host}:21114". For an ID server entered as a bare
IPv6 address such as 2001:db8::1, which test_if_valid_server accepts,
increase_port leaves it unchanged and the result is
http://2001:db8::1:21114. That does not parse as a URL (InvalidPort), so
login and every other API request fail.

Wrap the address in brackets in that branch only. Hostnames, IPv4 and
"[addr]:port" take the same path as before.

src/common.rs

@@ -1251,6 +1251,11 @@ fn get_api_server_(api: String, custom: String) -> String {
     if !s0.is_empty() {
         let s = crate::increase_port(&s0, -2);
         if s == s0 {
+            let s = if hbb_common::is_ipv6_str(&s) && !s.starts_with('[') {
+                format!("[{s}]")
+            } else {
+                s
+            };
             return format!("http://{}:{}", s, config::RENDEZVOUS_PORT - 2);
         } else {
             return format!("http://{}", s);
@@ -3209,6 +3214,14 @@ mod tests {
         assert!(!is_public("https://rustdesk.com@evil.test"));
     }
 
+    #[test]
+    fn test_api_server_from_bare_ipv6_rendezvous_server() {
+        assert_eq!(
+            get_api_server_("".to_owned(), "2001:db8::1".to_owned()),
+            "http://[2001:db8::1]:21114"
+        );
+    }
+
     #[test]
     fn test_should_use_tcp_proxy_for_api_url() {
         assert!(should_use_tcp_proxy_for_api_url(