Modernize Java SDK 3.0.0 and add playlist methods

[wenbinf]

Adds the five playlist write methods and prepares com.listennotes:podcast-api:3.0.0, while preserving all 25 existing public method names and parameter-map conventions. Endpoint wrappers, the standalone test contract, and README reference are generated from the canonical OpenAPI definition and Java registry in the Listen Notes monorepo.

Runtime and compatibility

• Requires Java 17+ and uses the JDK HTTP client. Requests keep per-client credentials/settings, preserve caller maps, encode nested path parameters, separate query/form fields, and retain explicitly empty notes and descriptions.
• Preserves existing constructors, JSON/response helpers, and the three legacy no-argument methods. getConnection(String) remains available as a deprecated compatibility helper.
• Retains status, response headers, exact error bodies, and underlying connection failures. All non-2xx responses throw, including 403 and redirects. Interruption restores the thread's interrupt flag.
• Upgrades Gradle, org.json, and JUnit with dependency locking and a checksummed wrapper. Replaces obsolete OSSRH/Dokka publishing with signed Central Portal artifacts.

Verification

• 46 offline tests passed on Java 17 and 25, including every generated method, encoding, empty fields, errors, concurrency, timeout, and interruption.
• 34 credential-free public mock integration tests passed.
• Standalone package build passed; all 31 README examples compile against the built JAR without executing requests.
• JAR, sources, Javadoc, generated POM, and Java 17 Gradle metadata validated.
• Monorepo generator, Django documentation, frontend typechecking, and snippet tests passed; website Java methods remain gated at published 1.1.6 until Maven Central publication is verified.

CI runs Java 17/25 offline checks and a separate public mock job. Package publication is separate; this PR does not publish to Maven Central.

Production read-only verification

Exercised all 21 GET methods through this SDK against the real v2 API:

• 19 returned HTTP 200 with valid JSON and usage/latency response headers, including both playlist read methods.
• spellcheck and fetchRelatedSearches returned HTTP 401 with the explicit API message requiring a PRO or ENTERPRISE plan. The SDK preserved those error responses correctly; successful responses for these two methods could not be verified with the provided key.
• No POST, PUT, or DELETE requests were sent to production.

GitHub CI passed: Java 17, Java 25, and the public mock integration job.