Add deletePlaylist support for PHP SDK 3.1.0

[wenbinf]

Adds $client->deletePlaylist(['id' => ...]) for DELETE /playlists/{id} and prepares the PHP SDK for 3.1.0. The generated README documents permanent deletion, the application confirmation requirement, and admin ownership restrictions.

The method uses the existing cURL transport and preserves string response bodies, getStatusCode(), and getHeaders(). Tests cover encoded and missing IDs, body-free DELETE requests, request-state isolation, API error details, redirects, retries, and timeouts. The public mock suite exercises the new endpoint, and CI verifies the standalone distribution includes it.

Validation

• PHP 8.4 and 8.5: 52 offline tests / 724 assertions passed on each.
• Public mock API: 35 integration tests / 434 assertions passed.
• Composer validation and lint passed; the standalone archive installs without development dependencies and exposes all 31 methods.
• SDK generation is deterministic; 52 synchronization tests and 17 Django documentation tests passed.

Dependencies and PHP requirements are unchanged. Publishing a tagged GitHub release is a separate step that triggers Packagist.