F5: web search/fetch on the Kimi services (kimi-cli parity)

web_search now speaks the Kimi search service (kimi-cli tools/web/search.py,
wire-verified against api.kimi.com):
- POST {coding_base}/search with {text_query, limit 1-20 (default 5),
  enable_page_crawling, timeout_seconds: 30}, OAuth bearer +
  X-Msh-Tool-Call-Id; results render in kimi-cli's Title/Date/URL/Summary
  schema with result URLs as citations.
- The old implementation called the xAI Responses API with a search model;
  that client is fully replaced and the entire model-based config surface
  is excised root-and-branch: web_search_model config keys/env/CLI plumbing,
  resolve_web_search_sampling_config, toolset web_search SamplerConfig,
  RemoteSettings.web_search_model, default_web_search_model.
- Enablement is now purely structural: the service exists only on the Kimi
  Code subscription channel, so OAuth sessions get Enabled and API-key-only
  sessions get Disabled (tool absent) — per PRD F5.

web_fetch gains the Kimi fetch service as its primary path (kimi-cli
tools/web/fetch.py): POST {coding_base}/fetch with {url}, Accept:
text/markdown, OAuth bearer + X-Msh-Tool-Call-Id; the 200 body is the
extracted markdown (still overflow-budgeted). Any service failure falls
back to the existing local pipeline (SSRF guards, cache, extraction).
The tool gate defaults ON now (kimi-cli always offers FetchURL) and the
egress User-Agent no longer claims grok-agent/x.ai.

Verified end-to-end against the scripted mock service: a headless session
drove web_search (limit/crawling/call-id observed on the wire) then
web_fetch (Accept + call-id observed) to completion.

crates/codegen/kigi-agent/src/builder.rs

@@ -2169,11 +2169,9 @@ mod tests {
         )
         .from_definition(definition)
         .with_web_search_config(WebSearchConfig::Enabled {
+            search_url: "https://api.kimi.example/coding/v1/search".into(),
             api_key: "test-key".into(),
-            base_url: "https://api.x.ai/v1".into(),
-            model: "test-web-search-model".into(),
             extra_headers: Default::default(),
-            alpha_test_key: None,
         })
         .with_web_fetch_config(WebFetchConfig::Enabled {
             params: Default::default(),
@@ -2295,11 +2293,9 @@ mod tests {
         use kigi_tools::notification::ToolNotificationHandle;
         let web_search_config = if web_search_enabled {
             WebSearchConfig::Enabled {
+                search_url: "https://api.kimi.example/coding/v1/search".into(),
                 api_key: "test-key".into(),
-                base_url: "https://api.x.ai/v1".into(),
-                model: "test-web-search-model".into(),
                 extra_headers: Default::default(),
-                alpha_test_key: None,
             }
         } else {
             WebSearchConfig::Disabled