Validate and compose provider endpoints

internal/domain/endpoint.go

@@ -0,0 +1,39 @@
+package domain
+
+import (
+	"errors"
+	"net/url"
+	"strings"
+)
+
+// NormalizeOpenAICompatibleBaseEndpoint trims and validates a source-neutral
+// OpenAI-compatible provider base endpoint.
+func NormalizeOpenAICompatibleBaseEndpoint(endpoint string) (string, error) {
+	endpoint = strings.TrimSpace(endpoint)
+	if endpoint == "" {
+		return "", errors.New("endpoint must not be blank")
+	}
+	if strings.Contains(endpoint, "#") {
+		return "", errors.New("endpoint must not contain a fragment")
+	}
+
+	parsed, err := url.Parse(endpoint)
+	if err != nil {
+		return "", errors.New("endpoint must be a valid URL")
+	}
+	parsed.Scheme = strings.ToLower(parsed.Scheme)
+	if parsed.Scheme != "http" && parsed.Scheme != "https" {
+		return "", errors.New("endpoint must use http or https")
+	}
+	if !parsed.IsAbs() || parsed.Hostname() == "" {
+		return "", errors.New("endpoint must be absolute and include a host")
+	}
+	if parsed.User != nil {
+		return "", errors.New("endpoint must not contain user information")
+	}
+	if parsed.RawQuery != "" || parsed.ForceQuery {
+		return "", errors.New("endpoint must not contain a query string")
+	}
+
+	return parsed.String(), nil
+}