Add deletePlaylist support for Swift SDK 3.1.0

[wenbinf]

Adds deletePlaylist(parameters:) for DELETE /playlists/{id} in both async/await and callback forms, and prepares PodcastAPI 3.1.0 for Swift Package Manager and CocoaPods. Applications can now permanently delete a playlist using the same client and response types as the other playlist operations.

The generated method reference explains that deletion is irreversible and that applications should ask for confirmation first. Generated wrappers, the independent test contract, README examples, and the example iOS app's CocoaPods lockfile cover all 31 methods. Existing Swift 6.0+, iOS 16+, macOS 13+, and Linux requirements remain unchanged.

Validation:

• Swift 6.0.3 on Linux and Swift 6.4 on macOS: builds with warnings as errors; all 15 offline tests pass.
• Independent package verification passes on both platforms; all 31 README examples compile.
• Both public mock integration tests pass, covering all 31 methods plus encoding and empty-value cases. No production API requests were sent.
• Deletion coverage includes async/callback dispatch, encoded identifiers, no query/body, invalid identifiers, HTTP errors, response headers, and cancellation.
• CocoaPods 1.16.2 validates iOS and macOS; the sample app resolves PodcastAPI 3.1.0.
• Monorepo generation is deterministic; all 52 synchronization tests, 17 Django documentation tests, and 84 frontend snippet/runner tests pass. Frontend typechecking passes.

Publication is separate. The monorepo's published Swift version remains 3.0.0 until both distribution channels are released and verified.