[jvm] Address kotlin compiler warnings

[varlanv]

Environment: Windows 10, IntelliJ 2025.1.1.1, no additional setup

Actual:
When I fork repository, and run gradle assemble task, I see many kotlin compiler warnings. As a non-maintainer, I have no way of knowing if those are expected, or there is some problem with my local setup. But by looking at these warnings, It seems like they will be present on any environment.

Expected:

1. gradle clean assemble finishes without kotlin compiler warnings
2. Optional: to prevent warnings pile up in the future, configure gradle/kotlin to treat warnings as errors, so that gradle assemble task fails if there are compiler warnings

[nedtwigg]

Good call, happy to look at a PR for this or I'll take a pass myself with claude code in the next week or two.