fix: restore image editing requests

Image editing requests included a generation-only response_format field that some compatible providers reject. Editing now sends only supported defaults while preserving explicitly configured custom request fields.\n\nRelease note: 修复部分图像服务商修改图片时报错的问题；参考图编辑现在可以正常提交。

src/main/agent/model-runtime.test.ts

@@ -5548,7 +5548,7 @@ describe('ModelAgentRuntime', () => {
     expect(form.get('prompt')).toBe('把背景改成白色')
     expect(form.get('n')).toBe('1')
     expect(form.get('quality')).toBe('medium')
-    expect(form.get('response_format')).toBe('b64_json')
+    expect(form.get('response_format')).toBeNull()
     expect(form.get('background')).toBe('transparent')
     const uploaded = form.get('image')
     expect(uploaded).toBeInstanceOf(Blob)

src/main/agent/model-runtime.ts

@@ -1793,7 +1793,12 @@ export class ModelAgentRuntime implements AgentRuntime {
     images: NonNullable<AgentExecutionRequest['images']>
   ): FormData {
     const form = new FormData()
-    const fields = this.createImageGenerationRequest(prompt)
+    const fields = this.createRequestBody({
+      model: this.options.model,
+      prompt: prompt.slice(0, 100_000),
+      n: 1,
+      quality: this.options.imageGenerationQuality ?? 'auto'
+    })
     for (const [name, value] of Object.entries(fields)) {
       form.append(
         name,