Add playlist support and modernize PHP SDK for 3.0.0

[wenbinf]

Adds the five playlist write methods for PHP SDK 3.0.0: createPlaylist, updatePlaylist, addPlaylistItem, deletePlaylistItem, and updatePlaylistItemNotes. All 25 existing method names, array arguments, and string response bodies are preserved. Method wrappers, a test contract, and compact README examples are generated from the Listen Notes API specification and PHP registry; this repository remains independently installable and testable.

The cURL transport now uses a fresh handle per request, preventing a POST or DELETE from affecting the next GET. It encodes path identifiers, separates query parameters and form fields, preserves empty notes/descriptions, accepts all 2xx responses, and reports HTTP 403 with PermissionDeniedException. Exceptions retain API error details and response metadata; header parsing preserves values containing colons. Redirects and automatic request retries are disabled.

Compatibility: requires PHP 8.4+. PHPUnit is upgraded to 13.3.3, with a refreshed Composer lockfile. The unused Composer plugin and Travis configuration are removed. GitHub Actions tests PHP 8.4 and 8.5, with public-mock integration in a separate job. Package publication remains a separate release step.

Validation:

• 50 offline tests / 616 assertions pass on PHP 8.4.25 and 8.5.10 using a local HTTP fixture and the actual cURL transport.
• 34 integration tests / 419 assertions pass against the public, stateless mock API without credentials.
• Composer validation and PHP syntax checks pass; dependency resolution reports no security advisories.
• The Composer archive excludes development dependencies and cache files. Its standalone installation passes the offline suite outside the checkout.
• Monorepo generation is deterministic; release-gate, Django documentation, and frontend checks pass. New PHP SDK snippets remain hidden until publication is verified.