fix: correct PlatformError variant in windows test

The protect_memory function returns MemoryProtectionFailed, not Internal.
This was causing a compilation error on Windows CI.

Co-Authored-By: Claude (glm-4.7) <noreply@anthropic.com>

Author: alphaqiu

tests/platform_test.rs

@@ -198,7 +198,7 @@ fn test_windows_length_validation() {
 
     let result = protect_memory(data.as_mut_ptr(), data.len());
     assert!(
-        matches!(result, Err(PlatformError::Internal { .. })),
+        matches!(result, Err(PlatformError::MemoryProtectionFailed(_))),
         "protect_memory should fail with invalid length on Windows"
     );
 }