fix: send messages in chat format in MCP add_memory tool

[brentkearney]

Description

The MCP add_memory tool in examples/mem_mcp/simple_fastmcp_serve.py passes memory_content as a plain string to the messages field in the /product/add API payload. The endpoint expects messages to be a list of chat message objects. When a plain string is passed, the SimpleStruct MemReader silently skips it — no error is raised, the API returns success, but no memory is actually stored.

The fix wraps the content as [{"role": "user", "content": memory_content}].

Related Issue (Required): Fixes #1274

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (does not change functionality, e.g. code style improvements, linting)
• Documentation update

How Has This Been Tested?

• Unit Test
• Test Script Or Test Steps (please provide)
• Pipeline Automated API Test (please provide)

Test steps:

1. Start MCP server with simple_fastmcp_serve.py
2. Call add_memory tool with test content
3. Call search_memories — verify memory is returned
4. Before fix: search returns empty. After fix: memory is stored and retrievable.

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

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