feat: 添加Gemini图像生成与处理功能

主要更新：

添加图像模型支持

新增MODEL_IMAGE配置项
在模型列表中添加gemini-2.0-flash-exp-image模型
修改ModelService以支持图像模型
增强图像处理能力

添加PicGoUploader类用于图像上传
实现图像响应处理逻辑(_extract_image_data)
支持base64图像数据的解码与上传
优化请求与响应处理

为图像模型添加特殊处理逻辑
修改API客户端以支持图像模型
更新GeminiRequest默认值
安全性调整

将TOOLS_CODE_EXECUTION_ENABLED默认设置为false

app/services/chat/api_client.py

@@ -28,6 +28,8 @@ class GeminiApiClient(ApiClient):
         timeout = httpx.Timeout(self.timeout, read=self.timeout)
         if model.endswith("-search"):
             model = model[:-7]
+        if model.endswith("-image"):
+            model = model[:-6]
         async with httpx.AsyncClient(timeout=timeout) as client:
             url = f"{self.base_url}/models/{model}:generateContent?key={api_key}"
             response = await client.post(url, json=payload)
@@ -40,6 +42,8 @@ class GeminiApiClient(ApiClient):
         timeout = httpx.Timeout(self.timeout, read=self.timeout)
         if model.endswith("-search"):
             model = model[:-7]
+        if model.endswith("-image"):
+            model = model[:-6]
         async with httpx.AsyncClient(timeout=timeout) as client:
             url = f"{self.base_url}/models/{model}:streamGenerateContent?alt=sse&key={api_key}"
             async with client.stream(method="POST", url=url, json=payload) as response: