缓存命中前保留后端校验取消

- BackendAssetService 在读取后端版本缓存前检查调用方取消令牌，避免缓存命中绕过取消。

- BackendAssetServiceTests 覆盖已有版本缓存时取消仍会传播的回归场景。

Author: C4AL

src/CodexCliPlus.Infrastructure/Backend/BackendAssetService.cs

@@ -118,6 +118,8 @@ private async Task<bool> IsExecutableVersionCurrentAsync(
         CancellationToken cancellationToken
     )
     {
+        cancellationToken.ThrowIfCancellationRequested();
+
         if (TryReadCachedExecutableVersion(executablePath, out var cachedVersion))
         {
             return IsExpectedBackendVersion(cachedVersion);

tests/CodexCliPlus.Tests/Backend/BackendAssetServiceTests.cs

@@ -173,6 +173,32 @@ public async Task IsExecutableVersionCurrentAsyncPropagatesCallerCancellation()
         Assert.IsAssignableFrom<OperationCanceledException>(exception);
     }
 
+    [Fact]
+    public async Task IsExecutableVersionCurrentAsyncPropagatesCallerCancellationBeforeCacheHit()
+    {
+        var pathService = new TestPathService(_rootDirectory);
+        await pathService.EnsureCreatedAsync();
+        var executablePath = Path.Combine(
+            pathService.Directories.BackendDirectory,
+            BackendExecutableNames.ManagedExecutableFileName
+        );
+        await File.WriteAllTextAsync(executablePath, "managed executable placeholder");
+        var service = new BackendAssetService(
+            new HttpClient(),
+            pathService,
+            new NullAppLogger(_rootDirectory)
+        );
+        await InvokeWriteVersionCacheAsync(service, executablePath, BackendReleaseMetadata.Version);
+        using var cancellation = new CancellationTokenSource();
+        cancellation.Cancel();
+
+        var exception = await Record.ExceptionAsync(() =>
+            InvokeIsExecutableVersionCurrentAsync(service, executablePath, cancellation.Token)
+        );
+
+        Assert.IsAssignableFrom<OperationCanceledException>(exception);
+    }
+
     public void Dispose()
     {
         if (Directory.Exists(_rootDirectory))