fix(test): Synchronize ranks before cleanup in VTK writer test

- Add MPI_Barrier before cleanup to prevent race conditions
- Only rank 0 performs cleanup to avoid concurrent file deletion
- Fixes vtk_writer_parallel test failure where rank 1's file was
  deleted by rank 0's cleanup before rank 1 could verify it

Root cause: Both ranks run test fixture destructor independently,
leading to concurrent cleanup. Rank 0 could delete all test_*.vti
files while rank 1 was still checking if its file exists, causing
the REQUIRE(fixture.file_exists(piece_filename)) assertion to fail.

Author: ahojukka5

tests/unit/io/test_vtk_writer.cpp

@@ -161,6 +161,12 @@ class VTKWriterTestFixture {
    * @brief Clean up all test files created during testing
    */
   void cleanup_test_files() {
+    // Synchronize all ranks before cleanup to avoid race conditions
+    MPI_Barrier(MPI_COMM_WORLD);
+
+    // Only rank 0 performs cleanup to avoid race conditions
+    if (m_rank != 0) return;
+
     // Pattern: test_*.vti, test_*.pvti
     for (const auto &entry : std::filesystem::directory_iterator(".")) {
       if (entry.is_regular_file()) {