fix(test): use _wfopen for UTF-8 path in attachment_ref_move test

The test creates a file using an absolute path from opts->run->run_path.
When UTF8_TEST_CWD=1, this path contains Thai characters that cannot be
represented in the Windows ANSI codepage, causing fopen() to fail. Use
_wfopen() with the wide-string path on Windows.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jpnurmi

tests/unit/test_envelopes.c

@@ -1052,7 +1052,11 @@ SENTRY_TEST(attachment_ref_move)
         = sentry__path_join_str(run_path, "test_minidump.dmp");
 
     size_t large_size = 100 * 1024 * 1024;
+#ifdef SENTRY_PLATFORM_WINDOWS
+    FILE *f = _wfopen(src_path->path_w, L"wb");
+#else
     FILE *f = fopen(src_path->path, "wb");
+#endif
     TEST_CHECK(!!f);
     fseek(f, (long)(large_size - 1), SEEK_SET);
     fputc(0, f);