fix: add JSON headers to DeepSeek requests and prevent string content from being parsed as file IDs in OpenAI adapter

internal/deepseek/client_completion.go

@@ -51,6 +51,7 @@ func (c *Client) streamPost(ctx context.Context, doer trans.Doer, url string, he
 	if err != nil {
 		return nil, err
 	}
+	headers = c.jsonHeaders(headers)
 	clients := c.requestClientsFromContext(ctx)
 	req, err := http.NewRequestWithContext(ctx, http.MethodPost, url, bytes.NewReader(b))
 	if err != nil {

internal/deepseek/constants.go

@@ -25,6 +25,7 @@ var defaultBaseHeaders = map[string]string{
 	"Host":              "chat.deepseek.com",
 	"User-Agent":        "DeepSeek/1.8.0 Android/35",
 	"Accept":            "application/json",
+	"Content-Type":      "application/json",
 	"x-client-platform": "android",
 	"x-client-version":  "1.8.0",
 	"x-client-locale":   "zh_CN",