feat: add GCS-backed session and conversation persistence

Enable session resumability across Cloud Run restarts:

- executor.ts: Save conversation history in task metadata during
  toSDKTask(), restore via setHistory() in reconstruct()
- gcs.ts: Persist conversation history as separate GCS object
  (conversation.tar.gz) alongside metadata and workspace
- session-store.ts: Add optional GCS-backed persistence with periodic
  flush and restore-on-startup for thread→session mappings
- handler.ts: Restore persisted sessions on initialize()
- types.ts: Add gcsBucket to ChatBridgeConfig
- app.ts: Pass GCS_BUCKET_NAME to chat bridge config

Validated end-to-end: message persists, Cloud Run restarts, follow-up
message in same thread correctly recalls prior context. Different
threads maintain isolation.

packages/a2a-server/src/http/app.ts

@@ -217,6 +217,7 @@ export async function createApp() {
         a2aServerUrl: chatBridgeUrl,
         projectNumber: process.env['CHAT_PROJECT_NUMBER'],
         debug: process.env['CHAT_BRIDGE_DEBUG'] === 'true',
+        gcsBucket: process.env['GCS_BUCKET_NAME'],
       });
       expressApp.use(chatRoutes);
       logger.info(