Avoid reordering when reading continuation state in AsyncOperation.SignalCompletion

[VSadov]

Fixes: #125015

Missing Read barrier.
The fix is roughly the same as in #72657, but in AsyncOperation

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[VSadov]

Also fixed the same issue in Microsoft.Bcl.AsyncInterfaces/src/System/Threading/Tasks/Sources/ManualResetValueTaskSourceCore.cs

Not sure if that code runs, but it uses the same pattern.

[Copilot]

Pull request overview

This PR fixes a missing acquire/read barrier in continuation-completion signaling to prevent theoretical (and observed) reordering issues when reading continuation dependencies, addressing sporadic System.Threading.Channels test failures.

Changes:

• Add Volatile.Read of _continuation in AsyncOperation.SignalCompletion to ensure continuation dependencies are observed in the correct order before invocation.
• Add a defensive Debug.Assert to document/enforce the expectation that _continuation is set when invoking it.
• Apply the same Volatile.Read pattern to the Microsoft.Bcl.AsyncInterfaces copy of ManualResetValueTaskSourceCore for consistency with the established fix pattern.

Reviewed changes

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

File	Description
src/libraries/System.Threading.Channels/src/System/Threading/Channels/AsyncOperation.cs	Adds an acquire read on _continuation before consuming _capturedContext / _continuationState, preventing reordering in SignalCompletion.
src/libraries/Microsoft.Bcl.AsyncInterfaces/src/System/Threading/Tasks/Sources/ManualResetValueTaskSourceCore.cs	Mirrors the acquire read pattern in SignalCompletion to avoid similar continuation-state reordering in the netstandard-compatible copy.

[VSadov]

Thanks!!