feat(context): 新增面向单一用户的跨上下文构建API

新增 `build_cross_context_for_user` 函数，提供一个简化的接口来构建指定用户的跨聊天上下文。

该函数会优先获取用户与机器人的私聊记录，然后聚合用户在其他群聊中的近期发言。通过整合这些信息，可以为需要了解用户近期跨聊天活动的插件提供一个便捷的数据来源。

src/plugin_system/apis/cross_context_api.py

@@ -4,6 +4,7 @@
 
 import time
 from typing import Any, TYPE_CHECKING
+from src.common.message_repository import find_messages
 
 from src.chat.message_receive.chat_stream import get_chat_manager
 from src.chat.utils.chat_message_builder import (
@@ -177,5 +178,93 @@ async def build_cross_context_s4u(
     logger.debug(f"[S4U] Successfully generated S4U context. Total length: {len(final_context)}.")
     return final_context
 
+async def build_cross_context_for_user(
+    user_id: str,
+    platform: str,
+    limit_per_stream: int,
+    stream_limit: int,
+) -> str:
+    """
+    构建指定用户的跨群聊/私聊上下文（简化版API）。
+    """
+    logger.debug(f"[S4U_SIMPLE] Starting simplified S4U context build for user {user_id} on {platform}.")
 
+    if not global_config or not global_config.cross_context or not global_config.bot:
+        logger.error("全局配置尚未初始化或缺少关键配置，无法构建S4U上下文。")
+        return ""
 
+    chat_manager = get_chat_manager()
+
+    private_context_block = ""
+    group_context_blocks = []
+
+    # --- 1. 优先处理私聊上下文 ---
+    private_stream_id = chat_manager.get_stream_id(platform, user_id, is_group=False)
+    if private_stream_id:
+        try:
+            user_ids_to_fetch = [str(user_id), str(global_config.bot.qq_account)]
+            messages_by_stream = await get_user_messages_from_streams(
+                user_ids=user_ids_to_fetch,
+                stream_ids=[private_stream_id],
+                timestamp_after=time.time() - (3 * 24 * 60 * 60),
+                limit_per_stream=limit_per_stream,
+            )
+            if private_messages := messages_by_stream.get(private_stream_id):
+                chat_name = await chat_manager.get_stream_name(private_stream_id) or "私聊"
+                title = f'[以下是您与"{chat_name}"的近期私聊记录]\n'
+                formatted, _ = await build_readable_messages_with_id(private_messages, timestamp_mode="relative")
+                private_context_block = f"{title}{formatted}"
+        except Exception as e:
+            logger.error(f"[S4U_SIMPLE] 处理私聊记录失败: {e}")
+
+    # --- 2. 处理其他群聊上下文 ---
+    streams_to_scan = [
+        stream_id for stream_id in chat_manager.streams
+        if stream_id != private_stream_id
+    ]
+
+    if streams_to_scan:
+        messages_by_stream = await get_user_messages_from_streams(
+            user_ids=[str(user_id)],
+            stream_ids=streams_to_scan,
+            timestamp_after=time.time() - (3 * 24 * 60 * 60),
+            limit_per_stream=limit_per_stream,
+        )
+
+        all_group_messages = []
+        for stream_id, user_messages in messages_by_stream.items():
+            if user_messages:
+                latest_timestamp = max(msg.get("time", 0) for msg in user_messages)
+                all_group_messages.append(
+                    {"stream_id": stream_id, "messages": user_messages, "latest_timestamp": latest_timestamp}
+                )
+
+        all_group_messages.sort(key=lambda x: x["latest_timestamp"], reverse=True)
+
+        remaining_limit = stream_limit - (1 if private_context_block else 0)
+        limited_group_messages = all_group_messages[:remaining_limit]
+
+        for item in limited_group_messages:
+            try:
+                chat_name = await chat_manager.get_stream_name(item["stream_id"]) or "未知群聊"
+                user_name = user_id # 简化处理
+                title = f'[以下是"{user_name}"在"{chat_name}"的近期发言]\n'
+                formatted, _ = await build_readable_messages_with_id(item["messages"], timestamp_mode="relative")
+                group_context_blocks.append(f"{title}{formatted}")
+            except Exception as e:
+                logger.error(f"[S4U_SIMPLE] 格式化群聊消息失败 (stream: {item['stream_id']}): {e}")
+
+    # --- 3. 组合最终上下文 ---
+    if not private_context_block and not group_context_blocks:
+        return ""
+
+    final_context_parts = []
+    if private_context_block:
+        final_context_parts.append(private_context_block)
+    if group_context_blocks:
+        group_context_str = "\n\n".join(group_context_blocks)
+        final_context_parts.append(f"### 其他群聊中的聊天记录\n{group_context_str}")
+
+    final_context = "\n\n".join(final_context_parts) + "\n"
+    logger.debug(f"[S4U_SIMPLE] Successfully generated context for user {user_id}. Total length: {len(final_context)}.")
+    return final_context