fix(chat): restore missing _explicit_web_intent definition (#5290)

chat_stream() references `_explicit_web_intent` in three places
(disabled-tools gating, global-disabled web allowance, and the
per-turn tool filter) but the assignment was dropped during a
branch merge. Every chat request raised

    NameError: name '_explicit_web_intent' is not defined

at routes/chat_routes.py, surfacing to the client as a bare
"Internal Server Error" before any LLM call was made — chat was
fully broken on dev and main.

Restore the original definition, computed from the already-derived
tool intent, immediately before its first use:

    _explicit_web_intent = bool(_tool_intent and _tool_intent.category == "web")

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

routes/chat_routes.py

@@ -876,6 +876,7 @@ def setup_chat_routes(
         # by default without having to send allow_bash in every request.
         if allow_bash is not None and str(allow_bash).lower() != "true":
             disabled_tools.add("bash")
+        _explicit_web_intent = bool(_tool_intent and _tool_intent.category == "web")
         if (
             allow_web_search is not None
             and str(allow_web_search).lower() != "true"