Add 'serper' as a web search backend, following the existing provider
pattern (keenable/exa): POST to https://google.serper.dev/search with the
X-API-KEY header, map the 'organic' results into the shared result format,
and fall back to DuckDuckGo when no key is configured.
- key resolved from config.api_key or SERPER_API_KEY env var
- 429 handled with a rate-limit message; other HTTP errors surfaced
- tests cover success, env-key, no-key fallback, HTTP error and rate limit
- docs: add Serper config example and list it in tools.web.search providers