[BUILD] Epic: Enhance Scala Compilation with Strict Compiler Warnings

[yew1eb]

Incrementally add strict Scala compiler flags to scala-maven-plugin (start with -Xfatal-warnings + existing -Ywarn-unused, then add others one by one) to catch bugs early, cut technical debt, and align with Apache best practices.

Sub Tasks

• [BUILD] Add -Xfatal-warnings and resolve existing -Ywarn-unused warnings #1870
• [BUILD] Add -Xlint:_ and resolve related linting warnings #1924
• [BUILD] Add advanced warn flags (-Ywarn-dead-code/-Ywarn-numeric-widen/-Ywarn-value-discard) and fix related warnings #1925

[yew1eb]

Please add the good first issue label to the subtasks. Thanks!

[yew1eb]

Please add the good first issue label to the subtasks. Thanks!

cc @richox