Fix streaming and file IO edge cases

Repair streaming callback/error handling and make file/session handling safer so the core API behaves correctly under real usage.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

dstalk-core/include/dstalk/dstalk_api.h

@@ -29,7 +29,7 @@ DSTALK_API void dstalk_set_model(const char* model);
 /* 同步对话: 发送 input，返回完整 AI 回复 (调用方通过 dstalk_free_string 释放) */
 DSTALK_API int  dstalk_chat(const char* input, char** output);
 
-/* 流式对话: 每收到一个 token 调用回调，回调返回 0 可提前取消 */
+/* 流式对话: 每收到一个 token 调用回调，回调返回 0 继续，非 0 取消 */
 typedef int (*dstalk_stream_cb)(const char* token, void* userdata);
 DSTALK_API int  dstalk_chat_stream(const char* input, dstalk_stream_cb cb, void* userdata);