NES xtab: gather language context and neighbor snippets concurrently instead of sequentially

[ulugbekna]

In XtabProvider.provideNextEdit (extensions/copilot/src/extension/xtab/node/xtabProvider.ts), the two context-gathering steps are awaited one after the other:

langCtx = await gatherLanguageContext();
if (cancellationToken.isCancellationRequested) { return new NoNextEditReason.GotCancelled('afterLanguageContextAwait'); }

neighborSnippets = await gatherNeighborSnippets();
if (cancellationToken.isCancellationRequested) { return new NoNextEditReason.GotCancelled('afterNeighborSnippetsAwait'); }

gatherLanguageContext() and gatherNeighborSnippets() are independent, so awaiting them sequentially adds their latencies together on the critical path before the prompt can be built. They should be started together (e.g. Promise.all) so they run concurrently.

Notes for whoever picks this up:

• This pattern appears in both the global-budget branch and the legacy/prod else branch, so both should be updated.
• Preserve the existing cancellation checks and their distinct GotCancelled reasons (afterLanguageContextAwait, afterNeighborSnippetsAwait) for telemetry continuity, checking cancellation after the combined await.
• Preserve the nLinesOfCurrentFileInPrompt telemetry timing in the prod branch (the current-file clip happens before these awaits there).

Surfaced during review of #323948.