fix(redaction): close forgery gap in event-level hook (v2.0.2)

[naji247]

Summary

Follow-up to #50 (redact_event event-level redaction hook), based on an adversarial code review of that change:

• Widened RESTORED_FIELDS — the event-level redact_event hook could previously forge or erase actor-identity fields (actor_id, identify_actor_given_id, identify_actor_name, identify_data), tags/properties, and client/server identity fields (client_name, client_version, server_name, server_version), even though those same fields are protected from the string-level redact_sensitive_information hook via PROTECTED_FIELDS. Now restored after the hook runs, same as id/session_id/project_id/event_type/timestamp.
• Stopped dict aliasing — the restored tags/properties/identify_data were the same dict objects as the original event, not copies; mutating one event's dict in place could silently corrupt another event sharing the reference. Now deep-copied.
• Tightened typing — _sync_event_redactor and apply_event_redaction now take RedactEventFunction and return Event | None instead of Callable[..., Any], so a hook that doesn't conform to the Event | None contract is a type-checker error, not a silent runtime surprise.
• Documented the contracts customers actually hit: the hook must mutate-and-return the Event it's handed (the return value replaces the event's fields wholesale, not as a diff — a partial/fresh Event silently nulls the rest), and the pydantic-Event return type is enforced by static typing only, not at runtime. Added to the docstrings on apply_event_redaction, the RedactEventFunction type alias, AgentCatOptions.redact_event, and the README example.
• Dropped an inaccurate doc claim — the docstring said the hook is dropped "or times out"; no timeout is implemented (or planned) on this hook's path, so removed the claim.
• Deduped id generation — the same if not event.id: event.id = generate_prefixed_ksuid(...) snippet was copy-pasted three times in event_queue.py's _process_event (one variant hardcoding "evt" instead of the EVENT_ID_PREFIX constant); extracted into one _ensure_event_id helper.
• Backfilled test coverage — fixed two tests that passed regardless of the behavior they claimed to check (an event_redaction_fn-clearing test whose mock fixture pre-baked the expected result, and a function-fields-exclusion test that couldn't detect a missing exclusion because the hook's input type never declares those fields anyway), and added coverage for SystemExit handling in the event-level hook, an async hook resolving to None, the original-event-unmutated invariant, a hook returning a non-Event value, client/server field protection, and the partial-Event field-nulling behavior. Every new/fixed test was mutation-tested against the behavior it covers.
• Bumped 2.0.1 → 2.0.2 (patch).

Test plan

• pytest tests/test_redaction.py tests/test_event_queue.py — all passing, including new coverage for actor/tags/properties/client/server forgery resistance
• pytest (full suite) — all passing (828 collected)
• mypy src/agentcat — no new errors vs. baseline (pre-existing, unrelated agentcat_api stub-package errors only)