Fetch an IPv6 source host written without a port (closes #295)

The fetcher took the host from the source URL and stripped a port with
net.SplitHostPort, which fails when there is no port, so a bracketed
IPv6 host such as [2001:db8::a] kept its brackets, its name lookup
failed, and the request answered 500. validateURL now takes the host
with url.URL.Hostname(), which drops the brackets and any port, so such
a host is fetched like one with a port, through /v1/image/ and through
an encrypted URL, and a private IPv6 address is still refused.

Model: opus-5-5

internal/httpfetcher/httpfetcher.go

@@ -473,16 +473,16 @@ func validateURL(ctx context.Context, rawURL string, allowHTTP bool) error {
 		return ErrUnsupportedScheme
 	}
 
-	// Parse to extract host
-	host := extractHost(rawURL)
-	if host == "" {
-		return ErrInvalidHost
+	parsedURL, err := neturl.Parse(rawURL)
+	if err != nil {
+		return fmt.Errorf("%w: %w", ErrInvalidHost, err)
 	}
 
-	// Remove port if present
-	h, _, err := net.SplitHostPort(host)
-	if err == nil {
-		host = h
+	// Hostname drops any port, and the brackets of an IPv6 address whether
+	// or not a port follows them
+	host := parsedURL.Hostname()
+	if host == "" {
+		return ErrInvalidHost
 	}
 
 	// Block obvious localhost patterns