Exp enable completions and NES fetcher

[chrmarti]

microsoft/vscode#270050

[Copilot]

Pull Request Overview

Adds experiment-based settings to control which fetcher implementation is used for completions and next edit suggestions, introduces tracking of failing (known-bad) fetchers, and extends fetch fallback logic to respect user-selected or disabled fetchers.

• Adds new experimental config keys and schema entries (completionsFetcher, nesFetcher).
• Extends fetch fallback pipeline to track failing fetchers and allow explicit fetcher selection with safeguards (disabled or known-bad).
• Passes selected fetcher through to next edit suggestions provider.

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/platform/networking/vscode-node/fetcherServiceImpl.ts	Stores and updates known-bad fetchers from fallback results.
src/platform/networking/test/node/fetcherFallback.spec.ts	Updates tests for new parameters and adds cases for useFetcher behavior.
src/platform/networking/node/fetcherFallback.ts	Adds config-aware fetcher selection, tracking of failing fetchers, and updated function signature.
src/platform/configuration/common/configurationService.ts	Introduces new experimental config keys for choosing fetchers.
src/extension/xtab/node/xtabProvider.ts	Passes experiment-based fetcher selection into streaming request options.
package.nls.json	Adds localized descriptions for new settings.
package.json	Declares new settings in configuration schema and updates completionsCore hash.

[Copilot]

Replacing the existing this._knownBadFetchers set with updatedKnownBadFetchers discards previously known bad fetchers on calls where only new failures are surfaced; this causes earlier failing fetchers to be forgotten unless they fail again in the same run. Merge instead: for (const f of updatedKnownBadFetchers) this._knownBadFetchers.add(f); to preserve cumulative state.

Suggested change

	this._knownBadFetchers = updatedKnownBadFetchers;
	for (const f of updatedKnownBadFetchers) {
	this._knownBadFetchers.add(f);
	}

[Copilot]

updatedKnownBadFetchers is initialized empty instead of cloning/unioning the incoming knownBadFetchers, and the caller later replaces its state (see FetcherService) leading to loss of previously known bad fetchers; either (a) initialize with new Set(knownBadFetchers) and add new ones, or (b) return only newly failed fetchers but have the caller union rather than replace to ensure consistent accumulation semantics.

Suggested change

	const updatedKnownBadFetchers = new Set<string>();
	const updatedKnownBadFetchers = new Set(knownBadFetchers);

[Copilot]

updatedKnownBadFetchers is initialized empty instead of cloning/unioning the incoming knownBadFetchers, and the caller later replaces its state (see FetcherService) leading to loss of previously known bad fetchers; either (a) initialize with new Set(knownBadFetchers) and add new ones, or (b) return only newly failed fetchers but have the caller union rather than replace to ensure consistent accumulation semantics.

[Copilot]

updatedKnownBadFetchers is initialized empty instead of cloning/unioning the incoming knownBadFetchers, and the caller later replaces its state (see FetcherService) leading to loss of previously known bad fetchers; either (a) initialize with new Set(knownBadFetchers) and add new ones, or (b) return only newly failed fetchers but have the caller union rather than replace to ensure consistent accumulation semantics.

[Copilot]

The parameter and return types use Set while the codebase introduces FetcherId; narrowing these to Set improves type safety and consistency with other parts (tests already use Set).

Suggested change

	export async function fetchWithFallbacks(availableFetchers: readonly IFetcher[], url: string, options: FetchOptions, knownBadFetchers: Set<string>, configurationService: IConfigurationService, logService: ILogService): Promise<{ response: Response; updatedFetchers?: IFetcher[]; updatedKnownBadFetchers?: Set<string> }> {
	if (options.retryFallbacks && availableFetchers.length > 1) {
	let firstResult: { ok: boolean; response: Response } | { ok: false; err: any } | undefined;
	const updatedKnownBadFetchers = new Set<string>();
	export async function fetchWithFallbacks(availableFetchers: readonly IFetcher[], url: string, options: FetchOptions, knownBadFetchers: Set<FetcherId>, configurationService: IConfigurationService, logService: ILogService): Promise<{ response: Response; updatedFetchers?: IFetcher[]; updatedKnownBadFetchers?: Set<FetcherId> }> {
	if (options.retryFallbacks && availableFetchers.length > 1) {
	let firstResult: { ok: boolean; response: Response } | { ok: false; err: any } | undefined;
	const updatedKnownBadFetchers = new Set<FetcherId>();