extension-sdk: openBrowserWindow gives no signal when the host refuses the URL #1776

What happens. extensionSDK.openBrowserWindow(url, target) returns void and sends UPDATE_LOCATION without waiting for an answer (extension_host_api.js, this.send(...)). When the URL is not covered by the manifest's new_window_external_urls, the host refuses it in its own frame — Uncaught (in promise) Error: External URL not matched in new_window_external_urls entitlement (verifyUpdateLocationRequest) — and nothing opens, but the extension gets no error and cannot tell the user why nothing happened.

Measured on Looker (original), Google-hosted, @looker/extension-sdk 26.20, Chrome: with only https://<instance>/auth* listed, openBrowserWindow('https://<other-host>/health') returns normally; the refusal appears only in the host frame's console. Adding https://<other-host>/* to new_window_external_urls makes the same call open the tab.

Request. A variant that resolves when the window opens and rejects with the host's reason when it is refused (for example openBrowserWindowAsync, or sendAndReceive for UPDATE_LOCATION), so an extension can show the user that a link was blocked by configuration. Context: we open Looker's own /auth this way for a backend-held OAuth sign-in (see #1772), where a silent refusal leaves the user waiting.