Modernize Go SDK and add playlist support for 3.0.0

[wenbinf]

Prepare version 3.0.0 with all five playlist write methods and generated endpoint wrappers, a contract snapshot, and README reference. Existing exported method names and response data/statistics remain available. The standalone module has no third-party dependencies and builds without the monorepo.

The transport now supports PUT, escapes path identifiers, separates query parameters from form fields, preserves empty notes/descriptions, and isolates each client's configuration. Responses retain HTTP status, headers, and body; API errors support errors.Is and errors.As.

Migration

• Requires Go 1.26+ and the github.com/ListenNotes/podcast-api-go/v3 import path.
• Custom HTTPClient implementations must add the five playlist methods.
• Replace direct sentinel error comparisons with errors.Is; default clients no longer follow redirects.

Validation

• Offline race tests, vet, build, formatting, and module checks pass on Go 1.26.8 and 1.27.1.
• All 30 method contracts and compile-only examples pass; 34 public mock requests pass.
• Standalone source copy and fresh consumer build successfully without dependency downloads.
• Monorepo generation, documentation tests, snippet tests, and frontend typechecking pass.
• CI covers the minimum and current stable Go versions, standalone packaging, and an opt-in public mock integration suite. Default tests never call production or the public mock.

Production smoke testing through this SDK covered all 21 GET endpoints: 19 returned HTTP 200, including both playlist reads; spellcheck and related searches returned the expected HTTP 401 PRO/ENTERPRISE plan restriction. Request paths/query encoding, User-Agent, response schema fields, quota/usage/latency/billing headers, and SDK error classification passed. The harness blocked non-GET requests and made no production writes.

All four GitHub CI jobs passed: minimum Go version, stable Go, standalone quality checks, and public mock integration. Publication and website snippet activation are separate release steps.