Validate schema_dir is non-empty for API scene source

Add early validation in create_scene_loader to throw a clear error
when schema_dir is empty and scenes.source='api', instead of failing
later with a confusing 'Failed to open scene schema' message.

Add corresponding unit test FactoryRequiresSchemaDir.

Author: Józef Daniecki

tracker/src/scene_loader.cpp

@@ -84,6 +84,11 @@ create_scene_loader(const ScenesConfig& config, const std::filesystem::path& con
             if (!manager_config.has_value()) {
                 throw std::runtime_error("Manager config is required when scenes.source='api'");
             }
+            if (schema_dir.empty()) {
+                throw std::runtime_error(
+                    "Missing required config: scenes.schema_dir (required when "
+                    "scenes.source='api')");
+            }
             return create_api_scene_loader(*manager_config, schema_dir);
         }
     }

tracker/test/unit/api_scene_loader_test.cpp

@@ -63,6 +63,18 @@ TEST(ApiSceneLoaderTest, FactoryRequiresManagerConfig) {
     EXPECT_THROW(create_scene_loader(config, "/tmp", std::nullopt, "/tmp"), std::runtime_error);
 }
 
+TEST(ApiSceneLoaderTest, FactoryRequiresSchemaDir) {
+    ScenesConfig config;
+    config.source = SceneSource::Api;
+
+    ManagerConfig mgr;
+    mgr.url = "https://localhost:443";
+    mgr.auth_path = "/tmp/nonexistent-auth.json";
+
+    // Empty schema_dir -> should throw with clear message
+    EXPECT_THROW(create_scene_loader(config, "/tmp", mgr, ""), std::runtime_error);
+}
+
 TEST(ApiSceneLoaderTest, FactoryCreatesLoaderWithManagerConfig) {
     ScenesConfig config;
     config.source = SceneSource::Api;