test: add unit tests for kotlinc/javac opts flag mapping

Tests all three code paths in convert._to_flags:
- value_to_flag dict lookup (warn, include_stdlibs, release)
- map_value_to_flag fn (jvm_target, x_backend_threads, x_optin, x_warning_level)
- derive.info path (x_suppress_warning, x_lint, add_exports, annotation_processor_options)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: restingbull

src/main/starlark/core/compile/worker/BUILD.bazel

@@ -0,0 +1,12 @@
+"""
+Assembles the kotlin builder binary.
+"""
+load()
+
+def _kotlinc_builder(ctx):
+
+
+
+kotlinc_builder = rule(
+
+)