Summary

Closes #696.

This adds lifecycle management for PgBuyerAgentRegistry mutation observers. Observers can now be removed explicitly, and the observer registry is protected by a lock so add/remove operations do not race with mutation notification setup.

What changed

• Added remove_mutation_observer(observer) -> bool.
• Guarded mutation observer registration/removal with a threading.Lock.
• Changed mutation notification to dispatch against a snapshot of observers, with callbacks still executed outside the lock.
• Documented snapshot semantics for observers added or removed while a notification is in flight.
• Added conformance coverage for unregistering observers and self-removal during notification.

Testing

• uv run ruff check src/adcp/decisioning/pg/buyer_agent_registry.py tests/conformance/decisioning/test_pg_buyer_agent_registry.py
• uv run mypy src/adcp/decisioning/pg/buyer_agent_registry.py
• ADCP_PG_TEST_URL=postgresql://postgres:pg@127.0.0.1:55432/postgres uv run python -m pytest tests/conformance/decisioning/test_pg_buyer_agent_registry.py -q

Note: uv run mypy src/adcp/decisioning/pg/buyer_agent_registry.py tests/conformance/decisioning/test_pg_buyer_agent_registry.py currently fails on the existing pytest.importorskip("psycopg_pool") typing pattern in the conformance test file, unrelated to this observer lifecycle change.