Add package license metadata

[yanziwei]

Summary

• add MIT license metadata to the package manifest
• align npm metadata with the repository license

Verification

• npm view unstructured-client@latest name version license repository homepage bugs --json
• node -e "const p=require('./package.json'); if (p.license !== 'MIT') throw new Error('license mismatch'); console.log(p.name, p.version, p.license)"
• git diff --check
• npm pack --dry-run --ignore-scripts --json