feat(decisioning): add registry observer removal

[sangilish]

Summary

Refs #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:[email protected]: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.

[sangilish]

This is ready for review now. CI is green, and I kept the change scoped to #696: observer removal, thread-safe add/remove, and snapshot notification semantics.

[bokelley]

Acknowledged — noted that CI is green and scope is contained to the observer lifecycle items from #696. Ready for human review.

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