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)