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>

config.example.toml

@@ -409,6 +409,7 @@ max_subagents = 10 # optional (1-20)
 #                            # baidu:      百度 AI Search via qianfan.baidubce.com，需 api_key
 #                            # volcengine: 火山引擎 Ark web_search (免费 2 万次/月), 需 api_key
 #                            #             也回退到 VOLCENGINE_API_KEY / VOLCENGINE_ARK_API_KEY / ARK_API_KEY 环境变量
+# base_url = "https://search.example/html/" # optional DuckDuckGo-compatible HTML endpoint
 # api_key = "YOUR_SEARCH_KEY" # required for tavily, bocha, and baidu; optional for metaso
 #                             # WARNING: treat config.toml like a secret file when
 #                             # storing API keys. Prefer env vars for local smoke tests.
@@ -416,6 +417,8 @@ max_subagents = 10 # optional (1-20)
 # Env-var overrides:
 #   DEEPSEEK_SEARCH_PROVIDER → search.provider
 #   DEEPSEEK_SEARCH_API_KEY  → search.api_key
+#   CODEWHALE_SEARCH_BASE_URL → search.base_url
+#   DEEPSEEK_SEARCH_BASE_URL  → search.base_url (legacy alias)
 #   METASO_API_KEY           → metaso key fallback
 #   BAIDU_SEARCH_API_KEY     → baidu key fallback