feat: abstract CubeView to Add & Search Handler

[CaralHsi]

Description

Summary:

1. 精简 AddHandler 主流程，只保留日志、参数整理（memory_content → messages）和统一入口，具体写入逻辑下沉到后续的 MemCubeView 等视图层。
   Simplified AddHandler.handle_add_memories to only handle logging, request normalization (memory_content → messages), and delegation, while moving actual write logic into cube view layers (e.g. MemCubeView).
2. 移除 AddHandler 内部的 _process_text_mem / _process_pref_mem / _get_sync_mode / _schedule_memory_tasks 等底层实现，使 Handler 不再直接操作 naive_mem_cube 与调度队列。
   Removed internal methods like _process_text_mem, _process_pref_mem, _get_sync_mode, and _schedule_memory_tasks from AddHandler, so the handler no longer directly manipulates naive_mem_cube or scheduling queues.
3. 保留并复用 _convert_content_messsage，继续支持仅传 memory_content 的调用路径，将其统一转换为标准 messages 结构。
   Kept and reused _convert_content_messsage to support the memory_content-only input path, consistently converting it into the standard messages format.
4. 在保持对外接口与返回结构不变的前提下，瘦身 AddHandler 以提升可维护性和可测试性，为后续引入多 Cube 视图（Single/CompositeCubeView）打基础。
   Preserved the external API behavior and response shape while slimming down AddHandler to improve maintainability and testability, paving the way for multi-cube view abstractions (e.g. SingleCubeView / CompositeCubeView).

Fix: #(issue)

Docs Issue/PR: (docs-issue-or-pr-link)

Reviewer: @(reviewer)

Checklist:

• I have performed a self-review of my own code | 我已自行检查了自己的代码
• I have commented my code in hard-to-understand areas | 我已在难以理解的地方对代码进行了注释
• I have added tests that prove my fix is effective or that my feature works | 我已添加测试以证明我的修复有效或功能正常
• I have created related documentation issue/PR in MemOS-Docs (if applicable) | 我已在 MemOS-Docs 中创建了相关的文档 issue/PR（如果适用）
• I have linked the issue to this PR (if applicable) | 我已将 issue 链接到此 PR（如果适用）
• I have mentioned the person who will review this PR | 我已提及将审查此 PR 的人