Fix Zipline leak after RealTreehouseApp.close call (#2897)

[adevone]

Fixes Zipline leak on RealTreehouseApp closure (#2897).

Cause of the leak

RealTreehouseApp.close indirectly schedules a resource unloading coroutine on dispatchers.zipline (RealTreehouseApp.stop -> CodeHost.stop -> CodeSession.stop). It starts working because of CoroutineStart.ATOMIC strategy but then crashes in ZiplineScope.close.

This exception could be seen by adding a breakpoint to CodeSession.handleUncaughtException:

java.lang.IllegalStateException: Zipline closed
	at app.cash.zipline.Zipline$endpoint$1.call(Zipline.kt:64)
	at app.cash.zipline.internal.bridge.OutboundCallHandler.callInternal$zipline_release(OutboundCallHandler.kt:116)
	at app.cash.zipline.internal.bridge.OutboundCallHandler.call(OutboundCallHandler.kt:75)
	at app.cash.redwood.treehouse.AppLifecycle$Companion$Adapter$GeneratedOutboundService.close(AppLifecycle.kt:26)
	at app.cash.zipline.ZiplineScope.close(ZiplineScope.kt:93)
	at app.cash.redwood.treehouse.ZiplineCodeSession.ziplineStop(ZiplineCodeSession.kt:73)
	at app.cash.redwood.treehouse.CodeSession$stop$1.invokeSuspend(CodeSession.kt:85)
	at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:34)
        at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:100)
  	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1156)
  	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:651)
  	at java.lang.Thread.run(Thread.java:1119)

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• [+] CHANGELOG.md's "Unreleased" section has been updated, if applicable.