websearch: task-level allowed_hosts extension for scope=code + JS-heavy extract evaluation (mock-first)

- search(..., allowed_hosts=[...]) extends (never replaces) the 15-domain
  code list for that call only; scope=general + allowed_hosts raises,
  empty/illegal values raise before any HTTP, dot-boundary matching,
  extension values never in error strings or attempted entries.
- Native-filter parity: Tavily/Exa get the effective list, Perplexity
  skips its native filter past its 20-domain cap (post-filter stays
  authoritative), Brave/SearXNG/DuckDuckGo rely on post-filtering.
- MCP web_search gains allowed_hosts passthrough (only when supplied,
  so older search_fn fakes keep working).
- JS-heavy evaluation documented in docs/web-search-mcp.md, mock-first:
  direct fetch is shell-only by construction; provider extract can
  supply rendered text in mock only; live SearXNG in this VM env
  failed (remote closed connection) and paid-backend live effects are
  unvalidated blockers, not claimed.
- Tests: test_code_search_ext (9) + test_js_heavy_extract (5 + 1 live
  skipped); existing websearch/code-search suites still green.

(cherry picked from commit 9c26bbe848438e1f34bcb000700aeb4c284f4f23)

cloud/websearch_mcp.py

@@ -42,7 +42,12 @@ _TOOLS = [
             "only (official language docs, GitHub, package "
             "registries, Stack Overflow, CVE/NVD); anything else "
             "is filtered out, never returned. Default scope "
-            "'general' is unfiltered. This tool does not do "
+            "'general' is unfiltered. With scope 'code' only, "
+            "allowed_hosts may name extra project domains for "
+            "this call (explicit allowlist extension, subdomains "
+            "included, dot-boundary matched); it is not an "
+            "arbitrary-site search, and it is rejected with "
+            "scope 'general'. This tool does not do "
             "shopping, social, places, travel, general news, "
             "weather, finance, or browser checkout."
         ),
@@ -61,6 +66,21 @@ _TOOLS = [
                                          "search. code: programming "
                                          "sources only, see tool "
                                          "description."},
+                "allowed_hosts": {"type": "array",
+                                  "items": {"type": "string"},
+                                  "description": "Only with scope "
+                                                 "'code': extra "
+                                                 "domains for this "
+                                                 "call only, on top "
+                                                 "of the builtin "
+                                                 "programming list. "
+                                                 "Omit for no "
+                                                 "extension; empty, "
+                                                 "wildcard, or "
+                                                 "non-hostname "
+                                                 "values are "
+                                                 "rejected, never "
+                                                 "silently ignored."},
             },
             "required": ["query"],
         },
@@ -139,11 +159,19 @@ def handle_request(request, search_fn=None, fetch_fn=None):
         args = params.get("arguments") or {}
         try:
             if name == "web_search":
+                # allowed_hosts is passed through only when the
+                # caller supplied it, so older search_fn fakes
+                # without that parameter keep working.
+                search_kwargs = {}
+                if "allowed_hosts" in args:
+                    search_kwargs["allowed_hosts"] = \
+                        args.get("allowed_hosts")
                 result = search_fn(
                     args.get("query"),
                     limit=args.get("limit", 5),
                     provider=args.get("provider", "auto"),
-                    scope=args.get("scope", "general"))
+                    scope=args.get("scope", "general"),
+                    **search_kwargs)
             elif name == "web_fetch":
                 result = fetch_fn(
                     args.get("url"),