fix: #3306 track MongoDB metadata timestamps

[Aphroq]

Summary

• Add created_at and updated_at fields to MongoDB session metadata documents on non-empty writes.
• Preserve created_at across later writes while refreshing updated_at and continuing to increment _seq atomically.
• Add fake MongoDB update support and regression coverage for first and subsequent writes.

Test plan

• uv run pytest tests/extensions/memory/test_mongodb_session.py -k "metadata_timestamps or add_and_get_items or multiple_add_calls_accumulate"
• uv run pytest tests/extensions/memory/test_mongodb_session.py
• uv run mypy tests/extensions/memory/test_mongodb_session.py
• uv run pytest tests/extensions/memory/test_mongodb_session.py -k metadata_timestamps
• bash .agents/skills/code-change-verification/scripts/run.sh

Issue number

Closes #3306

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass