Add missing typeguard exports for event and request type narrowing

[Copilot]

The TypeScript type definitions declared typeguards for events and requests (isApiGatewayEvent, isAlbRequest, etc.) but the JavaScript implementation only exported context typeguards. This caused runtime errors when using these functions.

Changes

• Added event typeguards (isApiGatewayEvent, isApiGatewayV2Event, isAlbEvent) to lib/typeguards.js
• Added request typeguards (isApiGatewayRequest, isApiGatewayV2Request, isAlbRequest) to lib/typeguards.js
• Exported all typeguards from main module in index.js
• Added null/undefined checks to all typeguard implementations to prevent runtime errors

Usage

import { isApiGatewayEvent, isAlbRequest } from 'lambda-api';

// Event type narrowing
if (isApiGatewayEvent(event)) {
  const sourceIp = event.requestContext.identity.sourceIp;
}

// Request type narrowing in middleware
api.use((req, res, next) => {
  if (isAlbRequest(req)) {
    const targetGroup = req.requestContext.elb.targetGroupArn;
  }
  next();
});

All typeguards now safely handle null/undefined inputs and correctly narrow TypeScript types based on the AWS Lambda event source (API Gateway, API Gateway v2, or ALB).

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