feat(widgets): add Exa web search + fix widget API endpoints (#1782)

* feat(widgets): add Exa web search + fix widget API endpoints

- Replace Tavily with Exa as primary stock-news search provider
  (Exa → Brave → SerpAPI → Google News RSS cascade)
- Add search_web tool to widget agent so AI can fetch live data
  about any topic beyond the pre-defined RPC catalog
- Exa primary (type:auto + content snippets), Brave fallback
- Fix all widget tool endpoints: /rpc/... paths were hitting
  Vercel catch-all and returning SPA HTML instead of JSON data
- Fix wm-widget-shell min-height causing fixed-size border that
  clipped AI widget content
- Add HTML response guard in tool handler
- Update env key: TAVILY_API_KEYS → EXA_API_KEYS throughout

* fix(stock-news): use type 'neural' for Exa search (type 'news' is invalid)

src/styles/main.css

@@ -19255,7 +19255,6 @@ body.has-breaking-alert .panels-grid {
 
 .wm-widget-shell {
   position: relative;
-  min-height: 100%;
   border: 1px solid color-mix(in srgb, var(--widget-accent, var(--accent)) 26%, var(--border));
   border-radius: 14px;
   background: