Fix indefinite wait in InitializeApplicationAsync when application terminates early

[Copilot]

InitializeApplicationAsync waits indefinitely for named pipe connection via GetUpdateCapabilitiesAsync. If the application terminates before establishing the pipe, Hot Reload hangs forever.

Changes:

• Cancellation propagation: Pass CancellationToken through session lifecycle, from TryCreatePendingSessionAsync → ApplyLaunchVariablesAsync → InitializeApplicationAsync
• Workaround for non-cancellable client: Use WithCancellation() in DeltaApplier.InitializeApplicationAsync to force cancellation since DefaultHotReloadClient.GetUpdateCapabilitiesAsync() ignores the token
• Race condition mitigation: Add ReentrantSemaphore to ProjectHotReloadSession to serialize StartSessionAsync/StopSessionAsync calls
• Process lifecycle handling: Set EnableRaisingEvents before subscribing to Process.Exited to avoid missing early termination events
• Session cleanup: Extract RemoveSessionState method and ensure StopSessionAsync is always called even when StartSessionAsync hasn't completed

Example flow:

// Session state owns cancellation
HotReloadSessionState sessionState = new(RemoveSessionState, _threadingService);

// Token propagates through initialization
await projectHotReloadSession.ApplyLaunchVariablesAsync(
    environmentVariables, 
    sessionState.CancellationToken);  // Was: default

// Process exit triggers cancellation
process.Exited += (sender, e) => DisposeSessionStateAndStopSession(sessionState);
// → sessionState.Dispose() → _cancellationTokenSource.Cancel()
// → InitializeApplicationAsync unblocks via WithCancellation()

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