Add delete_playlist support for Ruby SDK 3.1.0

[wenbinf]

Adds client.delete_playlist(id: ...) for DELETE /playlists/{id} and prepares podcast_api 3.1.0. The generated README documents permanent deletion, the application confirmation requirement, and admin ownership restrictions.

The method uses the existing transport and returns the usual HTTParty::Response. Coverage verifies encoded IDs, missing IDs, body-free DELETE requests, API error details, and no redirects or retries. The public mock integration suite includes the new endpoint, and CI checks that the installed gem exposes it.

Validation

• Ruby 3.3 and 4.0: 51 offline tests / 342 assertions passed on each.
• Public mock API: 35 integration tests / 436 assertions passed.
• Built and installed the gem outside the checkout; all 31 methods are present and all 51 offline tests passed against the installed package.
• SDK generation is deterministic; 52 synchronization tests and 17 Django documentation tests passed.

RubyGems publication remains separate.