fix(stealth): use Stealth class from playwright-stealth 2.x

[NaabZer]

Summary

Fixes #1959.

StealthAdapter._check_stealth_availability imported stealth_async / stealth_sync from playwright_stealth, which 2.x doesn't expose. Replaced with the Stealth class — apply_stealth_async accepts Page or BrowserContext, so existing call sites are unchanged.

Changes

• crawl4ai/browser_adapter.py — Swap the broken function-style imports for Stealth().apply_stealth_async(page).

Test plan

Ran the issue's repro against develop (broken) and this branch (fixed), with enable_stealth=False as the control:

version	enable_stealth=False	enable_stealth=True
develop	navigator.languages: ["en-US"]	navigator.languages: ["en-US"] ← stealth=True is a no-op
this branch	navigator.languages: ["en-US"]	navigator.languages: ["en-US","en"] ← Stealth init script applied

The added "en" is Stealth()'s default navigator_languages_override — proves the init script is now being injected. On Chromium 147 headless-shell most other classic signals (navigator.webdriver, chrome.csi/loadTimes, etc.) are already masked by the browser itself, so this is the clearest single-property A/B differentiator.

• Repro from [Bug]: enable_stealth=True is a silent no-op — StealthAdapter imports symbols that don't exist in playwright-stealth 2.x #1959 verified end-to-end against bot.sannysoft.com on both develop and this branch
• Confirmed Stealth.apply_stealth_async accepts Page (signature: Union[Page, BrowserContext]) — no call-site changes needed in browser_manager.py