Expose zod for SDK consumers to avoid version compatibility issues

[jmandel]

When building MCP tools, consumers need zod for input schemas but get type errors due to version mismatches with the SDK's internal zod dependency.

Problem:

import { z } from 'zod'; // Consumer's version (3.23.8)
server.registerTool('tool', {
  inputSchema: z.string() // Type error - incompatible with SDK's zod 3.25.76
});

Error: Type 'ZodString' is missing properties "~standard", "~validate"

Solutions

Option 1: Re-export from main index

// src/index.ts
export { z } from 'zod';

Usage: import { z } from '@modelcontextprotocol/sdk'

Option 2: Move zod to peerDependencies
Force consumers to install compatible zod version themselves.

Recommendation: Option 1 - simpler, no breaking changes, consumers get exact same zod version as SDK.