refactor: extract parseA2UIBlocks to lib/a2ui.ts; remove unused import re

- Extract parseA2UIBlocks() from A2UIRenderer.tsx into its own
  frontend/src/lib/a2ui.ts module to satisfy react-refresh/only-export-
  components ESLint rule (A2UIRenderer should only export React components)
- Update ChatPanel.tsx and test files to import from lib/a2ui
- Split ChatPanel.test.tsx mock: separate vi.mock for lib/a2ui
  (parseA2UIBlocks) and A2UIRenderer (component only)
- Remove unused 'import re' from tests/test_runtime_html.py (ruff F401)

All 216 frontend tests and 242 bash tests still pass. TypeScript and
Vite builds are clean. Remaining ESLint warnings (react-hooks/set-state-
in-effect in MCPAppRenderer and useSessions) are pre-existing patterns
not introduced by Phase 3.

frontend/src/components/A2UIRenderer.tsx

@@ -230,23 +230,4 @@ export function A2UIRenderer({ block, onAction }: A2UIRendererProps) {
   }
 }
 
-// ─── parseA2UIBlocks ─────────────────────────────────────────────────────────
 
-export function parseA2UIBlocks(content: string): A2UIBlock[] {
-  const blocks: A2UIBlock[] = []
-  const regex = /```a2ui\s*\n([\s\S]*?)```/g
-  let match: RegExpExecArray | null
-
-  while ((match = regex.exec(content)) !== null) {
-    try {
-      const parsed = JSON.parse(match[1])
-      if (parsed && parsed.type) {
-        blocks.push(parsed as A2UIBlock)
-      }
-    } catch {
-      // Silently skip invalid JSON
-    }
-  }
-
-  return blocks
-}