fix(client): release owned HTTP resources

[GregHolmes]

Summary

• make DeepgramClient and AsyncDeepgramClient AutoCloseable
• release dispatcher and connection-pool resources only when the SDK created the OkHttpClient
• preserve ownership of callers' .httpClient(...) instances
• document try-with-resources lifecycle management

The separate generated WebSocket teardown/concurrency work is tracked in #107.

Validation

• ./gradlew spotlessCheck test compileExamples --rerun-tasks
• Manual production Flux probe: connected a Listen v2 socket, disconnected it, then closed the parent client. The child JVM exited in 772 ms.

Closes #104