Use consistent multipart limit naming.

[samuel-williams-shopify]

Summary

• rename multipart structure constraints to use the measured quantity followed by _limit
• rename form-data field, upload, aggregate, and nesting constraints consistently
• retain total_size_limit for the aggregate across individually limited fields and uploads
• align constants, keywords, internal state, tests, documentation, and release notes

Depends on socketry/protocol-url#4 being merged and released. The packaged gem dependency remains unchanged.

Testing

Tested locally against the sibling protocol-url PR branch:

• bundle exec bake test
• bundle exec rubocop
• COVERAGE=PartialSummary bundle exec bake decode:index:coverage lib