Add deletePlaylist support for JavaScript SDK 3.1.0

[wenbinf]

Adds client.deletePlaylist({ id }) to the Node.js and Workers clients for DELETE /playlists/{id} and prepares version 3.1.0. The generated README documents permanent deletion and the developer's responsibility to add confirmation in their application's UI.

Regenerates the API contract and method reference, checks that missing playlist identifiers fail before sending a request, and exercises deletion through both transports against the public mock server.

Validation:

• Node: 47 tests passed; Workers: 45 tests passed; ESLint passed.
• Public mock integration: 26 tests passed across Node and Workers.
• SDK generation is deterministic and sync.py javascript --check passes.
• Packed npm artifact verified independently with both clients using offline responses.

Publication is separate. The monorepo keeps website SDK availability at 3.0.2 until 3.1.0 is published.