refactor(data): share server URL validation

The drawing library client needs the same base-address rule as the nest
repository. Extract it into one internal helper so both clients accept and
reject exactly the same addresses. Behavior is unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.Data/RemoteNestRepository.cs

@@ -38,14 +38,7 @@ public sealed class RemoteNestRepository : INestRepository, IDisposable
     {
         _httpClient = httpClient;
         _ownsClient = false;
-        var url = (baseUrl ?? "").Trim().TrimEnd('/');
-        if (!Uri.TryCreate(url, UriKind.Absolute, out var baseUri)
-            || (baseUri.Scheme != Uri.UriSchemeHttp && baseUri.Scheme != Uri.UriSchemeHttps))
-        {
-            throw new ArgumentException($"Invalid nest server URL: '{baseUrl}'", nameof(baseUrl));
-        }
-        // Keep the trailing slash so relative combines replace no path segment.
-        _baseUri = new Uri(baseUri.AbsoluteUri.TrimEnd('/') + "/");
+        _baseUri = ServerUrl.ParseBase(baseUrl, nameof(baseUrl));
     }
 
     private Uri Url(string relative) => new(_baseUri, relative);

OpenNest.Data/ServerUrl.cs

@@ -0,0 +1,21 @@
+namespace OpenNest.Data;
+
+/// <summary>Base-address validation shared by the nest server's HTTP clients.</summary>
+internal static class ServerUrl
+{
+    /// <summary>
+    /// Returns the absolute http(s) base address with exactly one trailing slash, so
+    /// relative combines replace no path segment. Throws for any other address.
+    /// </summary>
+    public static Uri ParseBase(string? baseUrl, string paramName)
+    {
+        var url = (baseUrl ?? "").Trim().TrimEnd('/');
+        if (!Uri.TryCreate(url, UriKind.Absolute, out var baseUri)
+            || (baseUri.Scheme != Uri.UriSchemeHttp && baseUri.Scheme != Uri.UriSchemeHttps))
+        {
+            throw new ArgumentException($"Invalid nest server URL: '{baseUrl}'", paramName);
+        }
+
+        return new Uri(baseUri.AbsoluteUri.TrimEnd('/') + "/");
+    }
+}