Fix Windows build: add default_CSPRNG and improve build.sh error handling

- Add Windows implementation of default_CSPRNG() using CryptGenRandom
- Add fallback stub for platforms without CSPRNG support
- Fix build.sh to properly detect build failures (use PIPESTATUS)
- Show proper error message instead of "Build completed successfully" on failure

tests/test_config_debug.c

@@ -44,7 +44,7 @@ static const char* config_content =
 
 // Create temp config file
 static int create_temp_config(void) {
-    if (test_mkdtemp(temp_dir) == NULL) {
+    if (test_mkdtemp(temp_dir) != 0) {
         fprintf(stderr, "Failed to create temp directory\n");
         return -1;
     }