fix(gemini-computer-use): remove unused getState call, use "about:blank" fallback by default

[dprevoznik]

The Gemini Computer Use API requires a URL in all function responses. Previously, the code attempted to call kernel.browsers.computer.getState() which doesn't exist in the SDK - the call would silently fail.

Changes:

1. Removed the non-existent getState() call from screenshot()
2. Always return 'about:blank' as the URL in both success and error cases
3. Added readme section explaining why (Computer Controls API doesn't provide page URL)

This works because:

• Gemini validates URL is present but relies on screenshots (couldn't find any information online about the model's usage of the url provided and it hasn't impacted my testing at all)
• 'about:blank' is a valid browser URI that Gemini accepts
• Tested and confirmed working with Gemini Computer Use API across python and typescript versions

---

Note

Ensures Computer Use function responses always include a URL and removes unsupported URL fetching.

• Always return url: about:blank from screenshot() and on action errors in both Python and TypeScript tools; loops now include url in error function_responses
• Remove non-existent getState calls from screenshot logic
• Add README "Known Limitations" explaining URL reporting fallback

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[Sayan-]

yeet