feat: add Google Chat bridge for A2UI integration

Implements a Google Chat HTTP webhook bridge that connects Google Chat
to the A2A server. Each Chat thread maps to an A2A contextId/taskId
pair. The bridge converts A2UI tool approval surfaces to Google Chat
Cards V2 with Approve/Always Allow/Reject buttons, and handles
CARD_CLICKED events to forward tool confirmations back to the A2A server.

Components:
- chat-bridge/types.ts: Google Chat event/response types
- chat-bridge/session-store.ts: Thread -> A2A session mapping
- chat-bridge/a2a-bridge-client.ts: A2A SDK client wrapper
- chat-bridge/response-renderer.ts: A2UI -> Google Chat Cards V2
- chat-bridge/handler.ts: Event handler (MESSAGE, CARD_CLICKED)
- chat-bridge/routes.ts: Express routes mounted at /chat/webhook

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

@@ -29,6 +29,7 @@ import { debugLogger, SimpleExtensionLoader } from '@google/gemini-cli-core';
 import type { Command, CommandArgument } from '../commands/types.js';
 import { GitService } from '@google/gemini-cli-core';
 import { getA2UIAgentExtension } from '../a2ui/a2ui-extension.js';
+import { createChatBridgeRoutes } from '../chat-bridge/routes.js';
 
 type CommandResponse = {
   name: string;
@@ -305,6 +306,22 @@ export async function createApp() {
       }
     });
 
+    // Mount Google Chat bridge routes if configured
+    const chatBridgeUrl =
+      process.env['CHAT_BRIDGE_A2A_URL'] || process.env['CODER_AGENT_PORT']
+        ? `http://localhost:${process.env['CODER_AGENT_PORT'] || '8080'}`
+        : undefined;
+    if (chatBridgeUrl) {
+      const chatRoutes = createChatBridgeRoutes({
+        a2aServerUrl: chatBridgeUrl,
+        debug: process.env['CHAT_BRIDGE_DEBUG'] === 'true',
+      });
+      expressApp.use(chatRoutes);
+      logger.info(
+        `[CoreAgent] Google Chat bridge enabled at /chat/webhook (A2A: ${chatBridgeUrl})`,
+      );
+    }
+
     expressApp.get('/tasks/:taskId/metadata', async (req, res) => {
       const taskId = req.params.taskId;
       let wrapper = agentExecutor.getTask(taskId);