Add deletePlaylist support for Java SDK 3.1.0

[wenbinf]

Add Client.deletePlaylist(Map<String, String>) for DELETE /playlists/{id} and prepare version 3.1.0. The generated README documents permanent deletion, developer-provided confirmation, ownership requirements, and the distinction between playlist references and the underlying podcasts/episodes.

The existing transport handles the operation without changes. Tests verify encoded IDs, an empty request body, untouched caller parameters, response headers, missing IDs, and 401/403/404/429/500 responses without SDK retries. Java 17 remains the minimum; existing methods and dependency versions are unchanged.

Validation:

• 48 offline tests pass on Java 17 and 25; all 32 README examples compile against the built JAR.
• 35 credential-free public-mock tests pass, covering all 31 methods and the deletion response.
• Standalone package verification passes outside Git/the monorepo; JAR, sources, Javadoc, and Maven POM build successfully.
• Deterministic generation and 52 synchronization tests, 17 Django documentation tests, frontend typechecking, and 90 documentation frontend tests pass.

No production API requests were made. Maven Central publication and advancing the website's published-version gate remain separate.