按参考图选择角色主形象的 provider 模型

角色主形象带参考图时走后端编辑端点，模型改用 gpt-image-2.5-sunburst-c

无参考图生成继续使用 gpt-image-2.5-flare-c

server-rs/crates/api-server/src/character_visual_assets.rs

@@ -36,8 +36,8 @@ use crate::{
     },
     http_error::AppError,
     openai_image_generation::{
-        DownloadedOpenAiImage, GPT_IMAGE_2_5_BUSINESS_NAME, GPT_IMAGE_2_5_GENERATION_MODEL,
-        OpenAiImageSettings, build_openai_image_http_client,
+        DownloadedOpenAiImage, GPT_IMAGE_2_5_BUSINESS_NAME, GPT_IMAGE_2_5_EDIT_MODEL,
+        GPT_IMAGE_2_5_GENERATION_MODEL, OpenAiImageSettings, build_openai_image_http_client,
         create_openai_image_generation_with_model, require_openai_image_settings,
     },
     platform_errors::map_oss_error,
@@ -946,10 +946,16 @@ async fn create_character_visual_generation_once(
     candidate_count: u32,
     reference_images: &[String],
 ) -> Result<GeneratedCharacterVisuals, AppError> {
+    // 中文注释：参考图存在时后端会走编辑端点，因此必须提交编辑模型而不是生成模型。
+    let provider_model = if reference_images.is_empty() {
+        GPT_IMAGE_2_5_GENERATION_MODEL
+    } else {
+        GPT_IMAGE_2_5_EDIT_MODEL
+    };
     let generated = create_openai_image_generation_with_model(
         http_client,
         settings,
-        GPT_IMAGE_2_5_GENERATION_MODEL,
+        provider_model,
         prompt,
         Some(build_character_visual_negative_prompt().as_str()),
         size,