test: cover mem_feedback basics

[snoopuppy582]

Fixes #1696

Summary

• Added a dedicated tests/mem_feedback package.
• Covered the base feedback interface contract and MemFeedbackConfig behavior.
• Added dependency-free utility tests for token estimation, update filtering, JSON extraction, chunking, and memory item creation.

Tests

• uv run --python 3.12 --extra mem-reader --with pytest pytest tests/mem_feedback -q
• uv run --python 3.12 --extra mem-reader --with pytest pytest tests/mem_feedback tests/mem_chat/test_base.py tests/mem_cube/test_base.py -q
• uv run --python 3.12 --extra mem-reader --with ruff ruff check tests/mem_feedback
• git diff --check

Notes

• Kept the tests local-only; no LLM, database, or network service is required.
• The contributing guide mentions a dev base branch, but this repository currently exposes main as the active/default base branch.