Include in WaitingWriteCount the thread that may be waiting in upgradable mode

[eduardo-vp]

Fixes #107077.

[Copilot]

Pull Request Overview

This PR updates the WaitingWriteCount property in ReaderWriterLockSlim so that it includes the thread potentially waiting in upgradable mode, addressing issue #107077.

• Added a comment explaining the inclusion of upgradable mode waiters.
• Adjusted the WaitingWriteCount property to sum _numWriteWaiters and _numWriteUpgradeWaiters.

Comments suppressed due to low confidence (1)

src/libraries/System.Private.CoreLib/src/System/Threading/ReaderWriterLockSlim.cs:1416

• Verify that _numWriteWaiters and _numWriteUpgradeWaiters are updated under the same concurrency control mechanism to prevent potential race conditions when summing their values.

public int WaitingWriteCount => (int)_numWriteWaiters + (int)_numWriteUpgradeWaiters;