Lock around access to s_dynamicSymbols

[tmat]

Alternative to #52883

[tmat]

@baronfel Alternative fix

[Copilot]

Pull request overview

This PR addresses thread-safety around the dynamic-completions metadata stored in DynamicSymbolExtensions by synchronizing access to the shared s_dynamicSymbols dictionary.

Changes:

• Add a static lock to guard reads/writes to s_dynamicSymbols.
• Consolidate the extension receiver to Symbol (instead of separate Option/Argument extension blocks), keeping IsDynamic functionality in one place.

[baronfel]

Thanks, this is more like what I was thinking based on how the other extensibility points solved concurrency.