fix: add root package export barrel

[cyphercodes]

Summary

• Add the missing root src/index.ts barrel so the existing package root export emits dist/esm/index.js and dist/cjs/index.js.
• Cover the root export surface used by the package entry point (Client, Server, McpError, protocol constants, and InMemoryTransport).

Fixes #971.

Testing

• npm run typecheck
• npm test -- test/index.test.ts
• npm run build
• npm run lint
• npm pack --json plus temp-project CJS require('@modelcontextprotocol/sdk') and ESM import ... from '@modelcontextprotocol/sdk' smoke tests
• git diff --check

[changeset-bot]

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Latest commit: d867ff8

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[pkg-pr-new]

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commit: d867ff8