Fix thread safety, null reference, and race conditions in SmartThreadPool

[Copilot]

Several major bugs in the SmartThreadPool codebase: missing locks on shared state, null dereferences on ThreadStatic fields, and a thread-unsafe Dispose pattern.

Changes

• SmartThreadPool.IsWorkItemCanceled — added null guards for CurrentThreadEntry and CurrentWorkItem (both can be null outside a thread pool thread); now throws InvalidOperationException with a clear message instead of crashing
• SmartThreadPool.ValidateWorkItemsGroupWaitForIdle — replaced re-access of CurrentThreadEntry.CurrentWorkItem with the already-captured local workItem; the property can be nulled by another thread between the null-check and the access
• WorkItemsGroup.Concurrency setter — wrapped read-modify-write of _concurrency in lock(_lock); without it, concurrent setters could compute a wrong diff and enqueue the wrong number of work items
• WorkItemsGroup.Start() and OnSTPIsStarting() — both read/write _isSuspended without holding _lock; concurrent calls could double-start or observe stale state
• SynchronizedDictionary.Count — the only property in the class that skipped lock(_lock); fixed for consistency and correctness
• WorkItemsQueue.Dispose() — moved _isDisposed = true inside the lock and after Cleanup(), preventing concurrent callers from both entering Cleanup()

// Before: NullReferenceException if called from a non-pool thread
public static bool IsWorkItemCanceled => CurrentThreadEntry.CurrentWorkItem.IsCanceled;

// After: clear contract enforcement
public static bool IsWorkItemCanceled
{
    get
    {
        if (CurrentThreadEntry == null || CurrentThreadEntry.CurrentWorkItem == null)
            throw new InvalidOperationException(
                "IsWorkItemCanceled can only be called from a work item executing in the SmartThreadPool");
        return CurrentThreadEntry.CurrentWorkItem.IsCanceled;
    }
}

Original prompt

Controleer de code op grote fouten

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