修正 nanobanana 空模型被误归一为 GPT 生成模型

- 新增 normalize_nanobanana_model，把空模型字符串统一回落到 gemini-3.1-flash-image-preview
- create_nanobanana_generate_content 改用该判据，空模型不再被 normalize_image_model 改写成 gpt-image-2.5-flare-c 后被 provider 校验拒绝
- nanobanana_generate_content_url 复用同一判据，去掉重复的空值分支
- 新增 nanobanana_model_resolution_defaults_blank_values_to_nanobanana 单测

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

@@ -99,6 +99,19 @@ pub fn normalize_image_model(model: &str) -> &str {
     }
 }
 
+/// nanobanana endpoint 只服务 Gemini 模型：空模型字符串回落到 nanobanana，
+/// 而不是 `normalize_image_model` 默认的 GPT 生成模型。
+///
+/// 中文注释：`nanobanana_generate_content_url` 与 `create_nanobanana_generate_content`
+/// 必须共用这一判据，否则空模型会先被归一成 GPT 生成模型、再被 provider 校验拒绝，
+/// 或用一个 GPT 模型名去拼 Gemini 的 generateContent URL。
+pub(crate) fn normalize_nanobanana_model(model: &str) -> &str {
+    match model.trim() {
+        "" => NANOBANANA_2_MODEL,
+        value => value,
+    }
+}
+
 /// Resolves a concrete provider model to its owning provider without rewriting the
 /// model string.
 ///
@@ -288,12 +301,7 @@ pub fn nanobanana_generate_content_url(settings: &ImageProviderSettings, model:
         .base_url
         .trim_end_matches("/v1")
         .trim_end_matches('/');
-    // 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,
-    };
+    let model = normalize_nanobanana_model(model);
     format!("{}/v1beta/models/{}:generateContent", base_url, model)
 }
 

server-rs/crates/platform-image/src/image_provider/runtime/executor.rs

@@ -24,7 +24,7 @@ use crate::image_provider::{
         build_image_edit_request_log_params, build_image_request_body_with_model,
         build_nanobanana_generate_content_request_body, images_edit_url, images_generation_url,
         nanobanana_generate_content_url, normalize_image_model, normalize_image_size_for_model,
-        resolve_image_provider,
+        normalize_nanobanana_model, resolve_image_provider,
     },
     response::handle_image_response,
     types::{GeneratedImages, ImageProvider, ImageProviderSettings, ReferenceImage},
@@ -262,7 +262,7 @@ pub async fn create_nanobanana_generate_content(
     reference_images: &[ReferenceImage],
     failure_context: &str,
 ) -> Result<GeneratedImages, PlatformImageError> {
-    let model = normalize_image_model(model);
+    let model = normalize_nanobanana_model(model);
     ensure_provider_matches_model(settings, model, failure_context)?;
     if reference_images.len() > VECTOR_ENGINE_NANOBANANA_MAX_REFERENCE_IMAGES {
         return Err(PlatformImageError::InvalidRequest {

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

@@ -5,6 +5,7 @@ use serde_json::json;
 
 use super::image_source::{decode_generated_image_base64, parse_reference_image_data_url};
 use super::payload::{extract_b64_images, extract_image_urls};
+use super::request::normalize_nanobanana_model;
 
 #[test]
 fn request_body_normalizes_size_prompt_and_candidate_count() {
@@ -58,6 +59,19 @@ fn provider_urls_normalize_root_and_v1_base_urls() {
     );
 }
 
+#[test]
+fn nanobanana_model_resolution_defaults_blank_values_to_nanobanana() {
+    assert_eq!(normalize_nanobanana_model("  "), NANOBANANA_2_MODEL);
+    assert_eq!(
+        normalize_nanobanana_model(" gemini-3.1-flash-image-preview "),
+        NANOBANANA_2_MODEL
+    );
+    assert_eq!(
+        normalize_nanobanana_model(GPT_IMAGE_2_5_EDIT_MODEL),
+        GPT_IMAGE_2_5_EDIT_MODEL
+    );
+}
+
 #[test]
 fn data_url_and_base64_image_decoding_preserves_image_metadata() {
     let data_url = format!(