feat(decisioning): add registry observer removal

[bokelley]

Summary

Replaces #764 so the required repo-owned checks can run with the normal base-repo secrets/check suite.

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.

Local validation

• PYTHONPATH=src ruff check src/adcp/decisioning/pg/buyer_agent_registry.py tests/conformance/decisioning/test_pg_buyer_agent_registry.py
• PYTHONPATH=src pytest tests/conformance/decisioning/test_pg_buyer_agent_registry.py -q (skips locally: no Postgres fixture in this workspace)

[aao-ipr-bot]

LGTM. Lock-protected snapshot copy with dispatch outside the lock is the right shape — re-entrant add/remove from inside an observer can't deadlock, and the "removal during in-flight notification still fires this round" contract is exercised by the self-remove test.

Things I checked

• Snapshot semantics at src/adcp/decisioning/pg/buyer_agent_registry.py:422-424 — tuple(self._mutation_observers) taken under the lock, iteration outside. Observers added mid-flight skip this round; observers removed mid-flight still fire this round. Matches the assertion sequence ["self-removing", "persistent", "persistent"] at tests/conformance/decisioning/test_pg_buyer_agent_registry.py:387.
• Re-entrancy: threading.Lock is non-reentrant, but the with block at _notify_mutation exits before iteration starts, so an observer calling remove_mutation_observer re-acquires cleanly. Self-remove test confirms.
• remove_mutation_observer return contract: list.remove()'s ValueError is caught → False; success → True. Idempotent double-remove asserted at test_pg_buyer_agent_registry.py:350-351.
• Conventional-commit prefix: feat(decisioning): is right — additive public method on PgBuyerAgentRegistry, no signature change, no ! needed.
• Lock scope is minimal — held only across append / remove / tuple-copy. Observer dispatch never runs under the lock.

Follow-ups (non-blocking — file as issues)

• with_caching at src/adcp/decisioning/pg/buyer_agent_registry.py:417 registers an anonymous lambda observer that cannot be removed by identity. Pre-existing, but now that removal is on the wire it's worth exposing the registered callback (or returning (cache, observer)) so callers can unwire. Out of scope here.

Minor nits (non-blocking)

1. Docstring on add_mutation_observer blurs the add vs. remove semantics. src/adcp/decisioning/pg/buyer_agent_registry.py:359-362 says "observers added or removed while a notification is in flight apply to the next mutation." The remove path is the opposite — a removed observer still fires this round (which is exactly what the self-remove test locks in). remove_mutation_observer's docstring at L374-376 gets it right; the add_mutation_observer blurb should match: "observers added during a notification fire on the next mutation; observers removed during a notification still fire for that in-flight notification but not subsequent ones."
2. Test could lock the N-registrations contract. tests/conformance/decisioning/test_pg_buyer_agent_registry.py:342-360 verifies double-remove returns False but doesn't assert the "one registration removed per call" promise at buyer_agent_registry.py:371-372. Two extra lines (add twice, remove once, mutate, expect one call) would pin that behaviorally. Optional.

A re-roll of #764 to get the repo-owned check suite — fine, but worth noting in the body so the next reviewer doesn't go hunting for the diff between them.

LGTM. Follow-ups noted below.