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_bgp_route_exchange.c

@@ -76,7 +76,7 @@ static int create_temp_directory(void) {
     }
     return -1;  // Failed after 100 attempts
 #else
-    return (test_mkdtemp(temp_dir) == NULL) ? -1 : 0;
+    return (test_mkdtemp(temp_dir) != 0) ? -1 : 0;
 #endif
 }
 

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;
     }

tests/test_etcp_100_packets.c

@@ -70,7 +70,7 @@ static const char* client_config_content =
 
 // Create temp config files
 static int create_temp_configs(void) {
-    if (test_mkdtemp(temp_dir) == NULL) {
+    if (test_mkdtemp(temp_dir) != 0) {
         fprintf(stderr, "Failed to create temp directory\n");
         return -1;
     }

tests/test_etcp_api.c

@@ -77,7 +77,7 @@ static const char* client_config_content =
 
 // Create temp config files
 static int create_temp_configs(void) {
-    if (test_mkdtemp(temp_dir) == NULL) {
+    if (test_mkdtemp(temp_dir) != 0) {
         fprintf(stderr, "Failed to create temp directory\n");
         return -1;
     }

tests/test_etcp_simple_traffic.c

@@ -68,7 +68,7 @@ static const char* client_config_content =
 
 // Create temp config files
 static int create_temp_configs(void) {
-    if (test_mkdtemp(temp_dir) == NULL) {
+    if (test_mkdtemp(temp_dir) != 0) {
         fprintf(stderr, "Failed to create temp directory\n");
         return -1;
     }

tests/test_pkt_normalizer_etcp.c

@@ -71,7 +71,7 @@ static const char* client_config_content =
 
 // Create temp config files
 static int create_temp_configs(void) {
-    if (test_mkdtemp(temp_dir) == NULL) {
+    if (test_mkdtemp(temp_dir) != 0) {
         fprintf(stderr, "Failed to create temp directory\n");
         return -1;
     }