Fix lost-wakeup race in WorkScheduler serial task wait

[michaelstaib]

Fixes #10051

[Copilot]

Pull request overview

This PR fixes a potential lost-wakeup race in the execution WorkScheduler when it waits for completion of a serial task (e.g., a mutation root field), and adds a regression test that stresses the specific concurrency window that previously could hang requests indefinitely.

Changes:

• Make the “completed check” and _signal.Reset() occur within the same _sync lock region while waiting for a serial task, preventing a lost wakeup.
• Refactor dispatch/completion logic into a locked TryDispatchOrCompleteUnsafe(...) helper to safely reuse logic from the wait path.
• Add an integration stress test that runs many concurrent mutation executions with an async child resolver that yields to exercise the race window.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/HotChocolate/Core/test/Execution.Tests/Integration/Spec/MutationTests.cs	Adds a high-concurrency regression test to detect hangs/timeouts caused by lost-wakeup behavior while awaiting serial mutation tasks.
src/HotChocolate/Core/src/Types/Execution/Processing/WorkScheduler.Execute.cs	Fixes the lost-wakeup window by performing completion visibility check + signal reset under the same lock; refactors dispatch/completion logic for safe reuse.

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