修正 nanobanana 端点空模型的回落值

nanobanana_generate_content_url 空模型回落到 nanobanana 模型

新增空模型路径的单元测试

server-rs/crates/platform-image/src/image_provider/protocol/request.rs

@@ -286,11 +286,13 @@ pub fn nanobanana_generate_content_url(settings: &ImageProviderSettings, model:
         .base_url
         .trim_end_matches("/v1")
         .trim_end_matches('/');
-    format!(
-        "{}/v1beta/models/{}:generateContent",
-        base_url,
-        normalize_image_model(model)
-    )
+    // 中文注释：nanobanana endpoint 只服务 Gemini 模型，空模型字符串应回落到
+    // nanobanana 模型，而不是 normalize_image_model 默认的 GPT 生成模型。
+    let model = match model.trim() {
+        "" => NANOBANANA_2_MODEL,
+        value => value,
+    };
+    format!("{}/v1beta/models/{}:generateContent", base_url, model)
 }
 
 pub(crate) fn build_image_edit_request_log_params(

server-rs/crates/platform-image/tests/image_provider.rs

@@ -211,6 +211,22 @@ fn nanobanana_generate_content_url_uses_model_path() {
     );
 }
 
+#[test]
+fn nanobanana_generate_content_url_blank_model_falls_back_to_nanobanana_model() {
+    let settings = ImageProviderSettings {
+        provider: ImageProvider::VectorEngine,
+        base_url: "https://vector.example".to_string(),
+        api_key: "test-key".to_string(),
+        request_timeout_ms: 1_000,
+        request_deadline: None,
+    };
+
+    assert_eq!(
+        nanobanana_generate_content_url(&settings, "  "),
+        "https://vector.example/v1beta/models/gemini-3.1-flash-image-preview:generateContent"
+    );
+}
+
 #[tokio::test]
 async fn image_edit_retries_send_timeout_once_and_succeeds() {
     let listener = TcpListener::bind("127.0.0.1:0")