feat: support configurable Serper base URL

[litescraper]

Summary

Support any Serper.dev-compatible endpoint (like litescrape.com, serpbase.dev, serpensapi.org, and others).

The Serper integration currently hardcodes its host. Allowing a configurable base URL lets deployments choose a compatible provider based on pricing and availability while retaining the existing search tools.

Set SERPER_BASE_URL to the provider?s base URL, without a search-type route, and configure its API key through the existing credentials. Trailing slashes are removed, and the integration appends /search, /images, /videos, or /news according to the requested search type.

When the setting is unset or empty, the integration continues using https://google.serper.dev. Document the setting in the README.

Validation

• Existing adjacent search suites: 47 passed.
• Type checking and focused ESLint passed.
• Local HTTP checks exercised the actual Serper implementation and response parsing for all four routes.
• An authenticated web search against a real Serper-compatible endpoint passed with safeSearch: 0.
• No tests added.

Compatibility

Package publishing and LibreChat dependency updates are outside this change.