Fixes #74.

Issue validity

Valid. EventBus.subscribe() schedules _ensure_subscription() with asyncio.ensure_future() after start, and _ensure_subscription catches every exception silently. The existing test test_subscribe_failure_logged_not_propagated explicitly locked in that behaviour. A listener could appear registered at the public API level while the underlying WebSocket subscribe never succeeded, so events would be silently missed.

Fix

Make subscription success/failure part of the API contract without breaking the existing fire-and-forget call sites.

• New async subscribe_async(event_type, handler) / async unsubscribe_async(event_type, handler) propagate transport errors. subscribe_async rolls back the newly registered handler if the first subscribe for an event type fails, so the caller's view never diverges from the transport state.
• The fire-and-forget subscribe() path now stores the scheduled task; callers can:
  • pending_subscription(event_type) -> awaitable in-flight subscribe task, or None.
  • subscription_failure(event_type) -> last recorded transport exception, cleared on a successful retry.
• start() preserves the historic 'never raise on subscribe failure' contract (a single flaky event type should not abort startup), but now records the failure for inspection.
• A done callback on the scheduled task moves the outcome (success / failure / cancellation) into the failure registry and clears the pending slot.

Tests

The existing test test_subscribe_failure_logged_not_propagated is replaced with test_subscribe_failure_during_start_is_recorded, which asserts the new contract. New coverage:

• test_subscribe_post_start_failure_is_observable
• test_subscribe_post_start_recovers_on_retry
• test_subscribe_async_propagates_failure_and_rolls_back
• test_subscribe_async_before_start_is_batched
• test_subscribe_async_additional_handler_skips_ws_call
• test_unsubscribe_async_propagates_transport_errors
• test_unsubscribe_async_unknown_handler_is_noop
• test_unsubscribe_async_does_not_release_ws_when_handlers_remain
• test_pending_subscription_none_when_idle
• test_cancelled_subscription_task_clears_pending

Checks run

• ruff check src tests -> clean
• ruff format --check src tests -> clean
• mypy src -> clean (strict)
• pytest tests/ --cov=haclient --cov-fail-under=95 -> 304 passed, 97.15% coverage

Backward compatibility

subscribe() / unsubscribe() / start() signatures and lifecycle are unchanged. Existing callers see no behavioural difference unless they opt in to subscribe_async / subscription_failure / pending_subscription.