Firefox-based stealth backend for BrowserToolkit (interest check)

hey, before building it i wanted to check if there'd be interest in an optional Firefox-based stealth backend for BrowserToolkit, parallel to the default Chromium-via-Playwright path. selected via an extra parameter, no change to defaults.

motivation: GAIA evaluation runs hit anti-bot walls on a growing share of target sites. related issues: #544 (anti-bot verification & MAX_TRY), #317 (auto captcha solve feature request), #427 (infinite scroll_down at the bottom of a page using BrowserToolkit). a Firefox build with fingerprint patches at the C++ source code level avoids the JS-shim detection surface that the standard playwright-stealth approach uses, so pages currently returning empty or 403 to the agent would render normally.

the backend wraps feder-cr/invisible_playwright, which drives a patched Firefox 150 binary (feder-cr/invisible_firefox, MPL-2.0, same license as Firefox upstream). two integration paths possible:

• a BrowserToolkit(backend='invisible_firefox') param (requires a small companion change in camel-ai/camel where the toolkit lives)
• an examples/run_invisible_firefox.py showing the wiring without touching upstream camel

also opened a draft PR #614 under community_usecase/Invisible-Firefox-Stealth-Backend/ with the proposal doc. discussions are disabled on this repo so opening the interest check here. if not in scope i'll close everything without noise.

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可选 Firefox 隐身后端，与默认 Chromium 路径并行。详细方案见 PR 中的英文说明。