Harden Accept header parsing

[samuel-williams-shopify]

Summary

• add coverage for quoted parameters, malformed input, wildcards, and quality factors
• support quoted pairs when splitting comma-separated media ranges
• require complete parameter consumption and validate wildcard and q forms
• preserve the existing regex-and-scan parser structure

Verification

• bundle exec sus test/protocol/http/header/accept.rb
• bundle exec bake test
• bundle exec rubocop lib/protocol/http/header/accept.rb test/protocol/http/header/accept.rb
• bundle exec bake decode:index:coverage lib