ZIP-558: updates the spec file to match the current SDK state

[ericlakich]

This pull request makes significant improvements to the API specification in docs/spec.yaml for the v6.0 models, focusing on better alignment with real API responses, improved field naming (including camelCase aliasing), and enhanced documentation for SDK developers. It also adds actual API response examples and new CI/CD quality requirements to ensure code quality and correctness.

Key changes:

Data Model & Field Mapping Improvements

• Added detailed implementation notes to clarify the use of Pydantic v2, field aliasing for camelCase API fields, and the optionality of many fields in real responses.
• Updated multiple model fields to reflect actual API behavior: made several fields optional, added api_field aliases for camelCase mapping, clarified that some fields are strings (not enums), and improved field descriptions and examples for clarity. [1] [2] [3] [4] [5]
• Replaced the origin_destination model with sourcing_rules to match the real API structure and field naming, including all necessary field aliasing and documentation. [1] [2]

Model Structure & Response Accuracy

• Refactored V60Metadata to nest response info under a new response object (V60ResponseInfo), accurately reflecting actual API responses.
• Added a new V60DisplayRate model and updated V60TaxSummary to include a display_rates array, matching the breakdowns in real responses.

Documentation & Testing

• Added a new section with actual API response examples to validate model structure and guide SDK development.
• Updated the test structure to require tests for HTTP client utilities and retry logic.

CI/CD & Code Quality

• Introduced a comprehensive quality_requirements section specifying minimum code coverage, type checking, formatting, linting, and Pydantic usage requirements, including sample configuration and code.

[linear]

ZIP-558 Establish SDK Foundation