Add unit tests for mem_feedback

[CaralHsi]

The mem_feedback module currently has no dedicated test coverage. This issue is to add basic unit tests for its public interfaces.

Target directory

• src/memos/mem_feedback/

Suggested test directory

• tests/mem_feedback/

Requirements

• Add tests for the base feedback interface and config behavior.
• Keep tests focused on local logic only.
• Do not require external services or network access.
• Follow the style of nearby module tests such as tests/mem_chat/ or tests/mem_cube/.

Acceptance Criteria

• A new tests/mem_feedback/ test package is added.
• Core public behavior is covered.
• Tests pass with pytest.