fix(ui): ensure GFM tables render in WebChat markdown (#20410)

- Pass gfm:true + breaks:true explicitly to marked.parse() so table
  support is guaranteed even if global setOptions() is bypassed or
  reset by a future refactor (defense-in-depth)
- Add display:block + overflow-x:auto to .chat-text table so wide
  multi-column tables scroll horizontally instead of being clipped
  by the parent overflow-x:hidden chat container
- Add regression tests for GFM table rendering in markdown.test.ts

ui/src/styles/components.css

@@ -1923,7 +1923,10 @@
   margin-top: 0.75em;
   border-collapse: collapse;
   width: 100%;
+  max-width: 100%;
   font-size: 13px;
+  display: block;
+  overflow-x: auto;
 }
 
 .chat-text :where(th, td) {