Add deletePlaylist support to Kotlin examples for SDK 3.1.0

[wenbinf]

Add Kotlin examples and documentation for deletePlaylist(mapOf("id" to playlistId)), using the published com.listennotes:podcast-api:3.1.0 dependency. The generated reference explains that deletion is permanent and applications should ask for confirmation before deleting a playlist.

CI now resolves Maven Central directly on Java 17 and 25 instead of checking out Java 3.0.0 source. The standalone verification script also works without a source override on macOS Bash 3; its previous empty-array expansion failed with set -u. An explicit source override remains available for development.

Validation:

• 33 offline tests pass on Java 17 and 25, including deletion response handling, encoded IDs, empty bodies, invalid IDs, and 404 responses.
• All 32 README examples compile; the application distribution builds with the verified Maven 3.1.0 JAR.
• 32 public-mock tests pass, making 34 credential-free requests across all 31 methods and empty-field/podcast-addition cases.
• Standalone checks pass with both Maven and an explicit Java source override.
• Generation is deterministic; 52 synchronization tests, 25 Django documentation tests, frontend typechecking, and 90 documentation frontend tests pass.

Java 17 remains the minimum. Kotlin, Gradle, JUnit, and their locked dependencies are unchanged. This repository publishes examples, not a separate Maven package. No production API requests were made.