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
-}

frontend/src/components/ChatPanel.tsx

@@ -4,7 +4,8 @@ import ReactMarkdown from 'react-markdown'
 import { IconSend } from '@tabler/icons-react'
 import { ToolCallCard } from './ToolCallCard'
 import { MCPAppRenderer } from './MCPAppRenderer'
-import { A2UIRenderer, parseA2UIBlocks } from './A2UIRenderer'
+import { A2UIRenderer } from './A2UIRenderer'
+import { parseA2UIBlocks } from '../lib/a2ui'
 import type { ChatMessage } from '../hooks/useChat'
 
 interface ChatPanelProps {

frontend/src/lib/a2ui.ts

@@ -0,0 +1,25 @@
+import type { A2UIBlock } from './types'
+
+/**
+ * Parses `a2ui` fenced code blocks out of a markdown-like content string.
+ * Each block must contain a valid JSON object with a `type` field.
+ * Invalid blocks are silently skipped.
+ */
+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
+}

frontend/src/test/A2UIRenderer.test.tsx

@@ -1,6 +1,7 @@
 import { describe, it, expect, vi } from 'vitest'
 import { render, screen, fireEvent } from '@testing-library/react'
-import { A2UIRenderer, parseA2UIBlocks } from '../components/A2UIRenderer'
+import { A2UIRenderer } from '../components/A2UIRenderer'
+import { parseA2UIBlocks } from '../lib/a2ui'
 import type { A2UIBlock } from '../lib/types'
 
 // Mock @tabler/icons-react so icon SVGs don't cause jsdom issues

frontend/src/test/ChatPanel.test.tsx

@@ -32,8 +32,8 @@ vi.mock('../components/MCPAppRenderer', () => ({
   ),
 }))
 
-// Mock A2UIRenderer module (component + parseA2UIBlocks)
-vi.mock('../components/A2UIRenderer', () => ({
+// Mock lib/a2ui parseA2UIBlocks parser
+vi.mock('../lib/a2ui', () => ({
   parseA2UIBlocks: vi.fn((content: string) => {
     const blocks: Array<Record<string, unknown>> = []
     const regex = /```a2ui\s*\n([\s\S]*?)```/g
@@ -48,6 +48,10 @@ vi.mock('../components/A2UIRenderer', () => ({
     }
     return blocks
   }),
+}))
+
+// Mock A2UIRenderer component
+vi.mock('../components/A2UIRenderer', () => ({
   A2UIRenderer: ({
     block,
     onAction,