Modernize Scala examples for Java SDK 3.0.0

[wenbinf]

The Scala examples currently use Java SDK 1.1.6 and have only a placeholder test. This update targets Java SDK 3.0.0 and adds generated Scala examples and README references for all 30 methods, including the five playlist write operations.

Scala remains an example application using com.listennotes:podcast-api; it does not introduce a separate transport or Maven package. The committed generated files let this repository build independently of the monorepo.

• Update Scala within the existing 2.13 line to 2.13.17, Gradle to 9.5.0, and JUnit to 6.1.2; require Java 17+, lock dependencies, and remove unused Guava and the old ScalaTest/JUnit 4 setup.
• Compile every README example and test all methods through loopback HTTP, including nested identifiers, encoding, empty fields, caller-map preservation, response headers, errors, and client isolation.
• Add separate credential-free public-mock integration tests and standalone distribution verification.
• Run CI on Java 17 and 25. While Java 3.0.0 is propagating on Maven Central, CI uses the reviewed Java commit b697b4026f0123820459427b3579b737b592d303 through an explicit Gradle composite build. A manual Maven-consumer job verifies the published artifact once available.

Validation: checks passed on Java 17 and 25, all 31 README examples compiled, 31 offline tests passed, and 31 integration tests made 33 successful public-mock requests. The standalone source build and application distribution also passed. Java 25 reports an upstream Zinc/protobuf sun.misc.Unsafe deprecation warning without failing checks. No production API requests or package publication were performed.