fix: make ITokenProvider.CreateHandler() public with proper XML docs

Fixes SA1615 (add <returns> XML doc to CreateHandler) and SA1518
(ensure file ends with exactly one newline). Also changes the method
visibility from internal to public as intended by PR #1001 to allow
custom ITokenProvider implementations outside the Zitadel.Net assembly.

Author: Copilot

src/Zitadel/Api/ITokenProvider.cs

@@ -34,5 +34,10 @@ static ITokenProvider ServiceAccount(
             options);
     }
 
-    internal DelegatingHandler CreateHandler();
+    /// <summary>
+    /// Creates the delegating handler for the token provider.
+    /// Made public to allow custom ITokenProvider implementations outside Zitadel.Net.
+    /// </summary>
+    /// <returns>A <see cref="DelegatingHandler"/> for the token provider.</returns>
+    public DelegatingHandler CreateHandler();
 }