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 {