fix: correct TryGetCachedResult doc and nullability

- Remove incorrect "Safe to call from any thread" claim; method reads
  EditorPrefs which requires the Unity main thread
- Add nullable return type annotation to match the null-on-cache-miss behavior

Author: Scriptwonder

MCPForUnity/Editor/Services/IPackageUpdateService.cs

@@ -14,9 +14,9 @@ public interface IPackageUpdateService
 
         /// <summary>
         /// Returns a cached update result if one exists for today, or null if a network fetch is needed.
-        /// Safe to call from any thread context but designed for main-thread use (reads EditorPrefs).
+        /// 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";