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

[Copilot]

PR #1001 introduced ITokenProvider.CreateHandler() as a public interface member but triggered two StyleCop build failures:

• SA1615 — CreateHandler() was missing a <returns> XML doc tag
• SA1518 — file had a trailing blank line (must end with exactly one newline)

Changes

• src/Zitadel/Api/ITokenProvider.cs
  • Added <returns> tag to CreateHandler() XML documentation
  • Removed extra trailing blank line

/// <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();