Add tool approval example: agent, backend, UI, and E2E tests (#226)

## Summary

Example implementation demonstrating the tool approval flow from #222:
- **Agent** (`example/convex/agents/approval.ts`): `delete_file` tool with `needsApproval: true`
- **Backend** (`example/convex/chat/approval.ts`): `sendMessage`, `submitApproval`, `handleApprovalDecision`
- **React UI** (`example/ui/chat/ChatApproval.tsx`): Approve/Deny buttons with denial reason input, type-safe approval helpers
- **E2E tests** (`example/convex/approval.test.ts`): Approve and deny flows through usageHandler

## Test plan

- [x] E2E approval test: approve flow executes tool and continues generation
- [x] E2E approval test: deny flow produces denial acknowledgment from model
- [x] Manual test: Send → Approve → clean completion (no stuck streaming)
- [x] Manual test: Send → Deny → model acknowledges denial

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: sethconvex

example/convex/_generated/api.d.ts

@@ -8,11 +8,13 @@
  * @module
  */
 
+import type * as agents_approval from "../agents/approval.js";
 import type * as agents_config from "../agents/config.js";
 import type * as agents_fashion from "../agents/fashion.js";
 import type * as agents_simple from "../agents/simple.js";
 import type * as agents_story from "../agents/story.js";
 import type * as agents_weather from "../agents/weather.js";
+import type * as chat_approval from "../chat/approval.js";
 import type * as chat_basic from "../chat/basic.js";
 import type * as chat_human from "../chat/human.js";
 import type * as chat_streamAbort from "../chat/streamAbort.js";
@@ -55,11 +57,13 @@ import type {
 } from "convex/server";
 
 declare const fullApi: ApiFromModules<{
+  "agents/approval": typeof agents_approval;
   "agents/config": typeof agents_config;
   "agents/fashion": typeof agents_fashion;
   "agents/simple": typeof agents_simple;
   "agents/story": typeof agents_story;
   "agents/weather": typeof agents_weather;
+  "chat/approval": typeof chat_approval;
   "chat/basic": typeof chat_basic;
   "chat/human": typeof chat_human;
   "chat/streamAbort": typeof chat_streamAbort;

example/convex/agents/approval.ts

@@ -0,0 +1,58 @@
+// See the docs at https://docs.convex.dev/agents/tool-approval
+import { Agent, createTool, stepCountIs } from "@convex-dev/agent";
+import { components } from "../_generated/api";
+import { defaultConfig } from "./config";
+import { z } from "zod/v4";
+
+// Tool that always requires approval
+const deleteFileTool = createTool({
+  description: "Delete a file from the system",
+  inputSchema: z.object({
+    filename: z.string().describe("The name of the file to delete"),
+  }),
+  needsApproval: () => true,
+  execute: async (_ctx, input) => {
+    return `Successfully deleted file: ${input.filename}`;
+  },
+});
+
+// Tool with conditional approval (requires approval for amounts > $100)
+const transferMoneyTool = createTool({
+  description: "Transfer money to an account",
+  inputSchema: z.object({
+    amount: z.number().describe("The amount to transfer"),
+    toAccount: z.string().describe("The destination account"),
+  }),
+  needsApproval: async (_ctx, input) => {
+    return input.amount > 100;
+  },
+  execute: async (_ctx, input) => {
+    return `Transferred $${input.amount} to account ${input.toAccount}`;
+  },
+});
+
+// Tool that doesn't need approval
+const checkBalanceTool = createTool({
+  description: "Check the account balance",
+  inputSchema: z.object({
+    accountId: z.string().describe("The account to check"),
+  }),
+  execute: async (_ctx, _input) => {
+    return `Balance: $1,234.56`;
+  },
+});
+
+export const approvalAgent = new Agent(components.agent, {
+  name: "Approval Demo Agent",
+  instructions:
+    "You are a helpful assistant that can delete files, transfer money, and check account balances. " +
+    "Always confirm what action you took after it completes.",
+  tools: {
+    deleteFile: deleteFileTool,
+    transferMoney: transferMoneyTool,
+    checkBalance: checkBalanceTool,
+  },
+  stopWhen: stepCountIs(5),
+  ...defaultConfig,
+  callSettings: { temperature: 0 },
+});