fix: Default basePath to / when server URL has no path

URI.create("http://127.0.0.1:4444").getPath() returns an empty
string, not null, so the previous Optional.ofNullable fallback
left basePath empty and HttpServer.createContext rejected it.

Author: thced

src/main/java/com/retailsvc/http/spec/Spec.java

@@ -245,7 +245,8 @@ private static String computeBasePath(List<Server> servers) {
     if (servers.isEmpty()) {
       throw new IllegalStateException("no servers declared");
     }
-    return Optional.ofNullable(URI.create(servers.getFirst().url()).getPath()).orElse("");
+    String path = URI.create(servers.getFirst().url()).getPath();
+    return (path == null || path.isEmpty()) ? "/" : path;
   }
 
   private static <T> Map<String, T> indexByRef(Map<String, T> components, String prefix) {

src/test/java/com/retailsvc/http/spec/SpecTest.java

@@ -33,6 +33,18 @@ void parsesMinimalSpec() {
     assertThat(spec.operations()).isEmpty();
   }
 
+  @Test
+  void basePathDefaultsToRootWhenServerUrlHasNoPath() {
+    Map<String, Object> raw =
+        Map.of(
+            "openapi", "3.1.0",
+            "info", Map.of("title", "x", "version", "1"),
+            "servers", List.of(Map.of("url", "http://127.0.0.1:4444")),
+            "paths", Map.of());
+    Spec spec = Spec.from(raw);
+    assertThat(spec.basePath()).isEqualTo("/");
+  }
+
   @Test
   void parsesPathsWithMethods() {
     Map<String, Object> raw =