feat: support configurable Serper base URL

[litescraper]

Summary

Allow users to point the existing Serper web-search integration at any Serper.dev-compatible endpoint (like litescrape.com, serpbase.dev, serpensapi.org, and others). This lets users choose another search provider or route requests through their own proxy without modifying the source.

Set SERPER_BASE_URL to the provider’s base URL, without /search, and SERPER_API_KEY to a key issued by that provider. The integration removes trailing slashes and appends /search, while retaining the existing authentication, request payload, and result parsing.

For example:

SERPER_BASE_URL=https://api.litescrape.com
SERPER_API_KEY=<your-provider-api-key>

When SERPER_BASE_URL is unset or empty, requests continue to use https://google.serper.dev/search.

The change adds the optional environment setting and documents its usage. No new dependencies are required.

Validation

• Focused Ruff, Black, and isort checks passed.
• Actual search helper HTTP checks passed against a local endpoint and the live Litescrape endpoint.
• Existing research-web suite: 2 passed, 2 failed. Both failures also occur on the unmodified base: stale Google-engine coverage and an exception expectation.
• No tests added.