Remove ScalaConverters from production code

[kaklakariada]

Review follow-up from PR #394 for issue #393.

ScalaConverters centralizes conversion between Java and Scala collections/options. After the production migration to Java, this keeps Scala types visible in production code.

Proposed direction:

• Replace ScalaConverters usages with regular Java types where APIs allow it.
• Keep narrowly scoped conversion only at unavoidable Scala dependency boundaries.
• Remove ScalaConverters once no production code depends on it.