ZIP-558: adds postalcode lookup function

[ericlakich]

This pull request adds support for retrieving sales and use tax rate details by postal code in the ZipTax API client. It introduces a new getRatesByPostalCode method, updates the OpenAPI spec and TypeScript models, provides example usage, and adds comprehensive tests for the new functionality.

New API endpoint and documentation:

• Added a new GetRatesByPostalCode endpoint to the OpenAPI spec (docs/spec.yaml), including parameter validation and response schema for postal code-based tax rate lookups.
• Documented the expected response structure (V60PostalCodeResponse, V60PostalCodeResult, V60PostalCodeAddressDetail) in the OpenAPI models section, and provided a real example response. [1] [2]

Client library enhancements:

• Implemented the getRatesByPostalCode method in ZiptaxClient, including parameter validation and HTTP request logic.
• Added TypeScript interfaces for the new endpoint’s request parameters and response types (GetRatesByPostalCodeParams, V60PostalCodeResponse, etc.). [1] [2]
• Updated imports to include new types.

Examples and testing:

• Extended the usage example in examples/basic-usage.ts to demonstrate how to call getRatesByPostalCode and display results.
• Added unit tests for getRatesByPostalCode, covering successful calls, parameter validation (format, length, required), and format options. [1] [2]

[linear]

ZIP-558 Establish SDK Foundation