Fix race condition causing NullReferenceException in LifecycleViewController

[Copilot]

Sporadic NullReferenceException in UpdateTaskName() during parallel service initialization, caused by Activity.Dispose() mutating _activities without holding _lock while LINQ iterates it under the lock.

List<T>.RemoveAt() zeros freed slots to null; a concurrent LINQ iterator could observe these null slots, causing x => x.FatalError is not null to throw.

Fix

• Activity.Dispose() now wraps _activities.Remove(this) in lock (_owner._lock), consistent with how StartTask already protects Insert

// Before
public void Dispose()
{
    if (FatalError is null)
        _owner._activities.Remove(this);  // unprotected — races with UpdateTaskName
    _owner.UpdateTaskName();
}

// After
public void Dispose()
{
    lock (_owner._lock)
    {
        if (FatalError is null)
            _owner._activities.Remove(this);
    }
    _owner.UpdateTaskName();
}

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