fix: remove nullable annotation to avoid CS8632 warning

UpdateCheckResult is a class — already nullable without the ? suffix.
The ? annotation requires #nullable enable which this project doesn't use,
causing CS8632 warnings across Unity versions.

Author: Scriptwonder

MCPForUnity/Editor/Services/IPackageUpdateService.cs

@@ -16,7 +16,7 @@ public interface IPackageUpdateService
         /// Returns a cached update result if one exists for today, or null if a network fetch is needed.
         /// Main-thread only (reads EditorPrefs).
         /// </summary>
-        UpdateCheckResult? TryGetCachedResult(string currentVersion);
+        UpdateCheckResult TryGetCachedResult(string currentVersion);
 
         /// <summary>
         /// Performs only the network fetch and version comparison (no EditorPrefs access).

MCPForUnity/Editor/Services/PackageUpdateService.cs

@@ -83,7 +83,7 @@ public UpdateCheckResult CheckForUpdate(string currentVersion)
         }
 
         /// <inheritdoc/>
-        public UpdateCheckResult? TryGetCachedResult(string currentVersion)
+        public UpdateCheckResult TryGetCachedResult(string currentVersion)
         {
             bool isGitInstallation = IsGitInstallation();
             string gitBranch = isGitInstallation ? GetGitUpdateBranch(currentVersion) : "main";