De-flake LoggerStartupSpec: gate the slow logger instead of racing a fixed delay

[Aaronontheweb]

LoggerStartupSpec.ActorSystem_should_start_with_loggers_timing_out(useAsync: True) failed on a 2-vCPU Windows agent with Timeout 00:00:03 while waiting for a message of type Akka.Event.Debug. It appeared in a build of AOT-stack PR #8608.

Not caused by the AOT stack

The stack changes LoggingBus.StartDefaultLoggers (#8603), so I checked that first:

• The Settings.cs diff between dev and the stack is empty.
• A custom logger like SlowLoggerActor falls through to the same Type.GetType call.
• The Task.WaitAll(logger-startup-timeout) block and the log messages it publishes are byte-identical.

Cause: publish before subscribe

1. SlowLoggerActor answers InitializeLogger after a fixed 1 s. logger-startup-timeout is 100 ms, so ActorSystem.Create gives up waiting and returns.
2. LoggingBus later publishes its one-shot Debug, "Logger … SlowLoggerActor … started", once the logger answers.
3. The test subscribes logProbe to the EventStream only after Create returns.
4. On the agent, Create stalled about 900 ms. The Debug went out at 53.582 and the subscription happened at about 53.583. The EventStream doesn't replay old messages, so the synchronous ExpectMsg<Debug> spent the whole AwaitAssert budget waiting.

useAsync: False has the same exposure, because akka.logger-async-start isn't read anywhere at runtime.

Change

• SlowLoggerActor now replies to InitializeLogger when a gate the test controls opens (Release.Task.PipeTo), not after a fixed delay.
• The gate stays shut for all of Create, so the logger still always misses the 100 ms window. That's the behavior under test.
• The test opens the gate only after it has subscribed, so the "started" Debug can't be published before anyone is listening. The finally block releases the gate.
• The two synchronous ExpectMsg calls inside AwaitAssertAsync become FishForMessageAsync with a 5 s bound, because both probes also receive other messages.
• The test still checks the same three things: Create doesn't throw, LoggingBus reports the slow logger started after the timeout, and the logger then receives log events.

An alternative was capturing the Debug with a LogFilterSetup. It wouldn't work here: the filter is only consulted by StandardOutLogger, and this spec replaces that with XUnitOutLogger.

Verification (local)

• LoggerStartupSpec passes 6/6, five runs in a row, about 2 s faster than before.

[Aaronontheweb]

LGTM

[Aaronontheweb]

more deterministic than the previous Task.Delay solution - LGTM