Fix race condition in channel completion

[stephentoub]

Fixes #123544

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[Copilot]

Pull request overview

This pull request fixes a race condition in UnboundedChannel<T> where ChannelReader.Completion could fail to complete even after all items were read and TryComplete() was called. The race occurred when TryRead dequeued the last item concurrently with TryComplete checking if the queue was empty, causing neither operation to trigger channel completion.

Changes:

• Added a recheck in TryComplete after releasing the lock to handle the race condition
• Added a regression test that reproduces the race condition with 100,000 iterations
• Included TaskTimeoutExtensions.cs helper for test timeout functionality

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/System.Threading.Channels/src/System/Threading/Channels/UnboundedChannel.cs	Added recheck logic in TryComplete to complete the channel if the queue becomes empty after releasing the lock, fixing the race condition
src/libraries/System.Threading.Channels/tests/UnboundedChannelTests.cs	Added regression test that reproduces the race condition by running concurrent TryRead and TryComplete operations
src/libraries/System.Threading.Channels/tests/System.Threading.Channels.Tests.csproj	Added reference to TaskTimeoutExtensions.cs for test timeout support