feat(search): harvest custom duckduckgo endpoint

Add optional [search].base_url support for DuckDuckGo-compatible private search endpoints, including a preferred CODEWHALE_SEARCH_BASE_URL env override and the legacy DEEPSEEK_SEARCH_BASE_URL alias.

Network policy now gates the configured endpoint host, custom endpoints do not fall back to public Bing, non-DuckDuckGo provider/base_url combinations and challenge pages return explicit errors, and custom endpoint results report the configured host as their source.

Fixes #2436

Reported by @Artenx

Harvested from PR #2510 by @cyq1017

Co-authored-by: cyq1017 <61975706+cyq1017@users.noreply.github.com>

docs/CONFIGURATION.md

@@ -1081,6 +1081,13 @@ parseable results. Bing remains selectable for users who explicitly want it,
 and Tavily, Bocha, Metaso, or Baidu can be selected when an API-backed provider
 is preferred.
 
+For a private/internal search service that serves DuckDuckGo-compatible HTML,
+keep `provider = "duckduckgo"` and set `base_url`; CodeWhale appends the `q`
+query parameter to that endpoint and applies network policy to its host.
+Custom endpoints do not fall back to public Bing. `CODEWHALE_SEARCH_BASE_URL`
+can override this per process; `DEEPSEEK_SEARCH_BASE_URL` remains accepted as
+the legacy alias.
+
 **Metaso** ([metaso.cn](https://metaso.cn)) has a 100 searches/day free quota;
 set `METASO_API_KEY` or `[search] api_key` for a higher quota.
 
@@ -1092,6 +1099,7 @@ only; it does not add a Baidu model provider.
 ```toml
 [search]
 provider = "baidu" # duckduckgo | bing | tavily | bocha | metaso | baidu
+# base_url = "https://search.example/html/" # optional with provider = "duckduckgo"
 # api_key = "YOUR_KEY" # required for tavily, bocha, and baidu; optional for metaso
 ```