Support customized system prompt base

[newmeta-ben]

Description

Added parameter to mos.chat(base_prompt=) to enable base_prompt to be customised on a per-query basis.

base_prompt (str, optional): A custom base prompt to use for the chat.

• It can be a template string with a {memories} placeholder.
• If not provided, a default prompt is used.

Issue: #90

Summary: (Support customized system prompt base)

Fix: #(90)

Reviewer: @fridayL

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 added necessary documentation (if applicable) | 我已添加必要的文档（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

[Copilot]

Pull Request Overview

Support per-query custom system prompts via a new base_prompt parameter

• Extend MOS.chat (in both CoT and non-CoT paths) and MOSCore.chat to accept and forward base_prompt
• Update _build_system_prompt to interpolate {memories} in a provided template and maintain backward-compatible behavior
• Add tests covering custom prompt usage in both core and high-level APIs

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/memos/mem_os/main.py	Updated MOS.chat, _chat_with_cot_enhancement, and _generate_enhanced_response_with_context to accept and forward base_prompt
src/memos/mem_os/core.py	Extended chat signature and enhanced _build_system_prompt to handle base_prompt templates and memory interpolation
tests/mem_os/test_memos_core.py	Added tests for MOSCore.chat with a custom base prompt and for _build_system_prompt behavior under various scenarios
tests/mem_os/test_memos.py	Added tests ensuring MOS.chat passes base_prompt correctly in both CoT-disabled and CoT-enabled modes

Comments suppressed due to low confidence (2)

src/memos/mem_os/main.py:57

• The docstring for _chat_with_cot_enhancement is missing a description for the new base_prompt parameter. Please add an Args entry explaining its purpose and usage.

    def _chat_with_cot_enhancement(

tests/mem_os/test_memos_core.py:595

• Consider adding a test that verifies when no base_prompt is provided to MOSCore.chat, the default system prompt ("You are a knowledgeable and helpful AI assistant.") is used and sent to the LLM.

    @patch("memos.mem_os.core.UserManager")