fix: support bare proxy format (user:pass@host:port) without scheme

Normalize bare proxy strings by prepending http:// before parsing when
@ is present but :// is absent. Tests added for Python and JS.

js/tests/puppeteer.test.ts

@@ -13,6 +13,7 @@ vi.mock("../src/download.js", () => ({
 
 vi.mock("../src/geoip.js", () => ({
   resolveProxyGeo: vi.fn().mockResolvedValue({ timezone: null, locale: null }),
+  maybeResolveGeoip: vi.fn().mockResolvedValue({}),
 }));
 
 describe("puppeteer launch", () => {