docs: add exported symbols comment above module.exports in background.js

[Copilot]

background.js exports 9 symbols via module.exports but had no comment documenting them, leaving consumers without a quick reference without reading the full file.

Changes

• background.js: Added a comment between /* istanbul ignore next */ and the module.exports guard listing all exported symbols and their context (Node.js/Jest-only)

/* istanbul ignore next */
// Exported for Node.js / Jest environments (not available in the browser extension context).
// Symbols: isNonRoutableHost, buildAcceptLanguage, DEFAULT_ACCEPT_LANGUAGE,
//          trackInitialHost, cleanupStaleTrackedRequests, initialHostByRequest,
//          redirectedRequestIds, REQUEST_TRACK_TTL_MS, MAX_TRACKED_REQUESTS
if (typeof module === "object" && module !== null) {

Style mirrors the inline comment convention used in utils.js. No behaviour change.