fix: update You.com Search endpoint to latest docs

[mouse-value-add]

Thanks again for reviewing this integration. I noticed the initial implementation used a legacy You.com Search endpoint shape, so this follow-up aligns with the latest docs from the official youdotcom-api skill.\n\n### What changed\n- Update default endpoint to \n- Use query param (instead of legacy )\n- Prefer env var (with kept as a backward-compatible alias)\n- Update README examples and function signature docs to match current API contract\n\n### Validation\n- \n\n### Compatibility\n- Existing users with continue to work.\n- Existing call sites remain unchanged unless they explicitly override endpoint/params.\n\nHappy to adjust naming or env-var strategy if you prefer stricter conventions.

[cursor]

PR Summary

Medium Risk
Medium risk because it changes the external HTTP contract (endpoint and query params) for the You.com search tool, which could break live search if the API differs; tests cover key formatting and legacy key fallback but don’t validate the real service.

Overview
Updates register_you_com_search_tool to use the new You.com Search endpoint (https://ydc-index.io/v1/search), send count instead of num_web_results, and clamp requested results once for request/formatting consistency.

Switches the default API key env var to YDC_API_KEY while supporting YOUCOM_API_KEY as a legacy alias when api_key_env is unchanged, and updates the README plus tests (including a new legacy-alias test) accordingly.

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[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

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