Add validation and null-forgiving operators to BasicAuth handler registration

- Added null-forgiving operators (!) to BasicAuthUsername and BasicAuthPassword
- Added validation to ensure both username and password are provided together
- Throws InvalidOperationException if only one credential is configured
- Prevents silently falling back to PassThroughHandler on misconfiguration
- Uses XOR operator (^) to detect partial configuration

This ensures users get clear error messages when BasicAuth is misconfigured
instead of silently failing to authenticate.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: Jaben

src/Gotenberg.Sharp.Api.Client/Extensions/TypedClientServiceCollectionExtensions.cs

@@ -87,11 +87,20 @@ public static IHttpClientBuilder AddGotenbergSharpClient(
             {
                 var ops = GetOptions(sp);
 
+                var hasUsername = !string.IsNullOrWhiteSpace(ops.BasicAuthUsername);
+                var hasPassword = !string.IsNullOrWhiteSpace(ops.BasicAuthPassword);
+
+                // Validate that both username and password are provided together
+                if (hasUsername ^ hasPassword)
+                {
+                    throw new InvalidOperationException(
+                        "BasicAuth configuration is incomplete. Both BasicAuthUsername and BasicAuthPassword must be set, or neither should be set.");
+                }
+
                 // Add basic auth handler if credentials are configured
-                if (!string.IsNullOrWhiteSpace(ops.BasicAuthUsername) &&
-                    !string.IsNullOrWhiteSpace(ops.BasicAuthPassword))
+                if (hasUsername && hasPassword)
                 {
-                    return new BasicAuthHandler(ops.BasicAuthUsername, ops.BasicAuthPassword);
+                    return new BasicAuthHandler(ops.BasicAuthUsername!, ops.BasicAuthPassword!);
                 }
 
                 // Return a pass-through handler if no auth is configured