Fix subscription disposal in Mono.asDeferred() extension

[Copilot]

The asDeferred() extension function subscribed to a Mono but didn't capture the Subscription reference, preventing proper cleanup and garbage collection after completion or error.

Changes:

• Store Subscription reference in subscriber state
• Clear reference in onComplete() and onError() to allow GC
• Use stored field consistently in cancellation handler

Before:

override fun onSubscribe(s: Subscription) {
    deferred.invokeOnCompletion {
        if (deferred.isCancelled) s.cancel()
    }
    s.request(Long.MAX_VALUE)
}

After:

private var subscription: Subscription? = null

override fun onSubscribe(s: Subscription) {
    subscription = s
    deferred.invokeOnCompletion {
        if (deferred.isCancelled) subscription?.cancel()
    }
    s.request(Long.MAX_VALUE)
}

override fun onComplete() {
    deferred.complete(value ?: throw NoSuchElementException(...))
    value = null
    subscription = null  // Allow GC
}

Addresses resource leak where subscriber held subscription reference indefinitely after terminal events.

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[Copilot]

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• repo.slne.dev
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