fix(web-search): support Brave llm-context date filters

docs/tools/brave-search.md

@@ -120,7 +120,7 @@ await web_search({
 - Each Brave plan includes **\$5/month in free credit** (renewing). The Search plan costs \$5 per 1,000 requests, so the credit covers 1,000 queries/month. Set your usage limit in the Brave dashboard to avoid unexpected charges. See the [Brave API portal](https://brave.com/search/api/) for current plans.
 - The Search plan includes the LLM Context endpoint and AI inference rights. Storing results to train or tune models requires a plan with explicit storage rights. See the Brave [Terms of Service](https://api-dashboard.search.brave.com/terms-of-service).
 - `llm-context` mode returns grounded source entries instead of the normal web-search snippet shape.
-- `llm-context` mode does not support `ui_lang`, `freshness`, `date_after`, or `date_before`.
+- `llm-context` mode supports `freshness` and bounded `date_after` + `date_before` ranges. It does not support `ui_lang`; `date_before` without `date_after` is rejected because Brave requires custom freshness ranges to include both start and end dates.
 - `ui_lang` must include a region subtag like `en-US`.
 - Results are cached for 15 minutes by default (configurable via `cacheTtlMinutes`).
 

docs/tools/web.md

@@ -296,7 +296,8 @@ show the `x_search` prompt.
 
 <Warning>
   Not all parameters work with all providers. Brave `llm-context` mode
-  rejects `ui_lang`, `freshness`, `date_after`, and `date_before`.
+  rejects `ui_lang`; `date_before` also needs `date_after` because Brave custom
+  freshness ranges require both start and end dates.
   Gemini, Grok, and Kimi return one synthesized answer with citations. They
   accept `count` for shared-tool compatibility, but it does not change the
   grounded answer shape.