fix(wait_event): drain signals with a blocking Receive loop (EN-1223) - #186

Problem (H3 — HIGH)

RunWaitEvent called channel.ReceiveAsync inside a workflow.Await predicate:

return workflow.Await(ctx, func() bool {
    var signal internalWorkflow.Event
    ok := channel.ReceiveAsync(&signal)
    ...
})

An Await predicate is evaluated only once per workflow-task wakeup. If a non-matching signal and the matching signal arrive in the same workflow task, the predicate consumes the non-matching one, returns false, and the matching signal stays buffered with nothing left to re-wake the coroutine — the stage blocks forever. Non-matching events are also silently destroyed.

Fix

Canonical blocking Receive loop that drains signals one at a time until the expected event arrives:

for {
    var signal internalWorkflow.Event
    channel.Receive(ctx, &signal)
    if signal.Name == waitEvent.Event { return nil }
}

Test

Adds a TestWaitEvent case signaling a non-matching event then the matching one in the same task; the stage completes (it would hang under the old implementation).

Severity: HIGH.