feat(copilot): opt-in HTTP cache for the Node fetch fetcher

[deepak1556]

Adds an opportunistic cache support to the Copilot Node fetch path. The cache is strictly opt-in per request and composes with the existing VSCode proxy and CA-injection patch.

• __vscodeCreateFetchPatch({ interceptors }) lets the extension host build a second proxy-aware fetch with extra undici interceptors. The default __vscodePatchedFetch is unchanged.
• NodeFetchFetcher builds an undici cache interceptor once at construction time and uses the factory to produce a cachedFetch that routes through both the proxy patch and the cache. Requests are tagged with an internal __copilotCachePatch marker (stripped before fetch); unmarked requests keep going through the regular patched fetch. When the host lacks the factory, caching is silently disabled so requests never bypass the proxy patch.
• FetchOptions.cache?: boolean — opportunistic hint. Fetchers without cache support ignore it; fallback to other fetchers is unaffected.
• Response.cacheStatus and FetchTelemetryEvent.cacheStatus: 'hit' | 'stale-hit' | 'revalidated' | 'miss' | 'bypass'.
• New setting github.copilot.advanced.debug.nodeFetchCache: 'off' | 'memory' | 'persistent' (default 'memory'). 'persistent' uses undici's SQLite store under the extension's global storage (undici-cache.v1.sqlite) when available, otherwise falls back to memory.
• New taggedCacheInterceptor wraps undici.interceptors.cache and stamps a private VSCODE_CACHE_STATUS_HEADER on the response so the base fetcher can read the outcome without parsing undici internals.
• BaseFetchFetcher exposes an overridable _buildRequestInit hook and reports cacheStatus on Response and fetchTelemetry.

Notes

• No behavior change for callers that don't set cache: true.
• The cache interceptor is constructed once per fetcher instance; the composed dispatcher chain is reused so connection pooling is preserved.
• Depends on feat: add fetch interceptor options vscode-proxy-agent#100

For #308310

[Copilot]

Pull request overview

Adds opt-in HTTP caching support to Copilot’s Node fetch path (undici-based) while ensuring cached requests still route through VS Code’s proxy/CA-injection patching, and surfaces cache outcomes via Response.cacheStatus plus fetch telemetry.

Changes:

• Exposes a host-provided __vscodeCreateFetchPatch({ interceptors }) factory to compose additional undici interceptors on top of the existing proxy-aware fetch patch.
• Adds an opt-in cache hint (FetchOptions.cache?: boolean) with cache status propagation (Response.cacheStatus, telemetry) and a debug setting to choose cache mode (off/memory/persistent).
• Introduces a taggedCacheInterceptor wrapper that stamps cache outcome onto responses for downstream attribution.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/vs/workbench/api/node/proxyResolver.ts	Exposes a fetch-patch factory on globalThis to allow composing additional undici interceptors while preserving proxy/CA patching.
extensions/copilot/src/platform/telemetry/vscode-node/telemetryServiceImpl.ts	Adds cacheStatus to fetch telemetry payload/classification and emits it as metadata.
extensions/copilot/src/platform/networking/vscode-node/test/fetcherServiceCrash.spec.ts	Updates tests for the new FetcherService constructor dependency.
extensions/copilot/src/platform/networking/vscode-node/fetcherServiceImpl.ts	Wires cache mode setting and persistent store location into NodeFetchFetcher, and forwards cacheStatus into fetch telemetry events.
extensions/copilot/src/platform/networking/node/test/taggedCacheInterceptor.spec.ts	Adds unit tests for cache-status tagging and classification behavior.
extensions/copilot/src/platform/networking/node/taggedCacheInterceptor.ts	Implements the cache interceptor wrapper that stamps cache outcome onto response headers.
extensions/copilot/src/platform/networking/node/nodeFetchFetcher.ts	Adds opt-in cache routing via a request marker and composes the cache interceptor using the host fetch-patch factory.
extensions/copilot/src/platform/networking/node/baseFetchFetcher.ts	Adds a request-init hook and reads stamped cache status to populate Response.cacheStatus.
extensions/copilot/src/platform/networking/common/fetcherService.ts	Extends fetch options and response/telemetry types with cache-related fields.
extensions/copilot/src/platform/configuration/common/configurationService.ts	Adds the advanced.debug.nodeFetchCache setting (off/memory/persistent).

Comments suppressed due to low confidence (2)

extensions/copilot/src/platform/networking/node/taggedCacheInterceptor.ts:86

• readHeader returns only the first element when the header value is a string[]. For headers like warning this can drop additional warnings and lead to incorrect stale-hit classification. Consider checking all values (and/or joining) when the value is an array.

	if (typeof value === 'string') {
		return value;
	}
	if (Array.isArray(value) && typeof value[0] === 'string') {
		return value[0];

extensions/copilot/src/platform/networking/node/taggedCacheInterceptor.ts:94

• stampStatus skips stamping when undici supplies headers in raw wire format (string[]). In that case BaseFetchFetcher will always report cacheStatus: 'bypass' even if the cache served the response. Consider supporting the array form by appending the header key/value pair (and/or normalizing to an object) before forwarding to the downstream handler.

function stampStatus(headers: unknown, status: CacheStatus): void {
	if (headers && typeof headers === 'object' && !Array.isArray(headers)) {
		(headers as Record<string, string>)[VSCODE_CACHE_STATUS_HEADER] = status;
	}

[chrmarti]

LGTM!