Clean up remaining build and compiler warnings #3339

The build succeeds, but compilation still produces warnings in application code, instrumentation tests, and generated Safe Args sources. Gradle also reports deprecated API usage in build plugins.

Scope:

• Remove redundant null checks, non-null assertions, casts, and exhaustive when branches.
• Resolve parameter naming and Java field shadowing warnings.
• Investigate Safe Args warnings about casts that only succeed for null; address the generator or its configuration rather than editing generated files.
• Investigate Gradle deprecations reported for KSP and grgit.
• Preserve the intentional Toast deprecation marker in BaseTest unless the underlying limitation is resolved.