CmdPal: Refine superseding value gate lifecycle #51380

Summary of the Pull Request

This PR tefines SupersedingAsyncValueGate<T> so requests arriving as the worker becomes idle complete reliably, and disposal prevents later results from being applied.

• Preserves latest-request-wins behavior and keeps caller cancellation scoped to waiting.


• Serializes request scheduling and keeps caller cancellation scoped to its wait.


• Runs work cancellation callbacks asynchronously and drains them before reporting idle.


• Adds asynchronous disposal while allowing committed application to finish.


• Documents lifecycle guarantees and exposes gate and request state in the debugger.

PR Checklist

• [x] Closes: #51379
  <!-- - [ ] Closes: #yyy (add separate lines for additional resolved issues) -->


• [ ] Communication: I've discussed this with core contributors already. If the work hasn't been agreed, this work might be rejected


• [ ] Tests: Added/updated and all pass


• [ ] Localization: All end-user-facing strings can be localized


• [ ] Dev docs: Added/updated


• [ ] New binaries: Added on the required places
  
  
  • [ ] JSON for signing for new binaries
  
  
  • [ ] WXS for installer for new binaries and localization folder
  
  
  • [ ] YML for CI pipeline for new test projects
  
  
  • [ ] YML for signed pipeline
    
  
  
  
  


• [ ] Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed