fix(memory): preserve observation timestamp in extraction

[haosenwang1018]

Summary

• Pass metadata timestamp into the additive extraction prompt so historical conversations use the correct Observation Date.
• Fall back to created_at when timestamp is not present.
• Cover both sync and async extraction paths.

Closes #4963.

Testing

• uv run --with pytest --with pytest-asyncio --with pytest-mock --with ruff python -m pytest tests/memory/test_main.py -q
• uv run --with ruff ruff check mem0/memory/main.py tests/memory/test_main.py