Join a subscription's handler before releasing the caller's reference

[lalinsky]

A smaller fix for the autounsubscribe async basic functionality flake. It replaces #179.

The bug

An async subscription that reaches its autounsubscribe limit calls removeSubscriptionInternal on its own handler task, which drops the connection's reference. If the caller has already called sub.deinit(), that reference is the last one. The release runs destroy(), which joins the handler task group from inside that same task, so it never returns. Connection.deinit() then panics with 1 live subscription(s).

The fix

Subscription.deinit() closes the queue and joins the handler before it unsubscribes and drops the caller's reference. The handler can then never be the one that drops the last reference. destroy() still runs the same close and cancel, which do nothing on the second call.

Unlike #179, this leaves live_subscriptions and the panic for unreleased subscriptions unchanged. The other library code that drops subscription references runs on connection tasks that close() joins before that check.

Calling sub.deinit() from the subscription's own handler is documented as not allowed. It was already a self-join in most cases.

Testing

• autounsubscribe tests: 7 failures in 10 runs on main, 0 in 50 runs with this change.
• Full ./check.sh passes.

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• src/subscription.zig

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