Fix semantic highlighting cancellation during rapid typing

[Copilot]

Rapid typing could cancel completion work in a way that left semantic token delta requests without a response, which prevented VS Code from updating semantic highlighting for newly typed text.

• Request pipeline

  • Remove serial processing from PsesCompletionHandler registration so completion cancellations no longer block the semantic tokens pipeline.
  • Keep completion requests interruptible without coupling them to DidChangeTextDocument sequencing.

• Cancellation handling

  • Catch TaskCanceledException in completion-item resolve and return the original item instead of letting cancellation escape through the LSP pipeline.
  • Preserve the existing completion response shape when the underlying command lookup is canceled.

• Regression coverage

  • Add a test for the completion resolve cancellation path to verify the handler returns cleanly under cancellation.
  • Update the test to use CancelAsync() to avoid synchronous cancellation analyzer violations.

try
{
    CommandInfo commandInfo = await CommandHelpers.GetCommandInfoAsync(..., cancellationToken);

    if (commandInfo is not null)
    {
        return request with
        {
            Documentation = await CommandHelpers.GetCommandSynopsisAsync(
                commandInfo, _executionService, cancellationToken)
        };
    }
}
catch (TaskCanceledException)
{
    return request;
}