Modernize PodcastAPI 3.0.0 and add playlist methods

[wenbinf]

Prepare PodcastAPI 3.0.0 with all five playlist write methods and Swift 6 support. Existing method names and callback overloads remain available, with async/await overloads for all 30 operations.

Changes

• Generate endpoint wrappers, an independent test contract/dispatcher, and the README reference from the monorepo's canonical OpenAPI spec and Swift registry.
• Encode path, query, and form values separately, support PUT, preserve empty notes/descriptions, and retain HTTP error details and case-insensitive response headers.
• Keep client configuration isolated, support cancellation, reject redirects, and fix Linux JSON comparison and session-cleanup issues.
• Preserve PodcastAPI as the Swift package, product, module, and CocoaPods name. Update the podspec and add Swift Package Index documentation configuration.
• Replace default live API tests with offline coverage; add separate public-mock integration tests and CI for minimum/current Swift, standalone builds, README examples, and CocoaPods.

Compatibility

Requires Swift 6.0+, iOS 16+, or macOS 13+; also supports Linux. Responses are immutable, callbacks use Sendable closures, and exhaustive error switches need the new permission-denied and unexpected-response cases. There are no external runtime package dependencies.

The version is prepared as 3.0.0. This PR does not publish a release or submit the repository to Swift Package Index. The monorepo's website gate remains at the published 1.1.8 baseline until release verification.

Validation

• All 14 offline tests passed on Swift 6.0.3/Linux and Swift 6.4/macOS, covering every async and callback method.
• All 34 credential-free public-mock requests passed; no production requests were sent.
• Independent package builds and all 30 generated README examples compiled on both compiler/platform combinations.
• CocoaPods validation passed for iOS and macOS; the updated iOS sample built with Swift 6.
• Monorepo generator tests (40), Django documentation tests (14), frontend typechecking, and snippet tests (46) passed.