Add validation and test coverage for migrated sanitizer behavior

[nradakovic]

Why

The sanitizer migration changes ownership boundaries and integration flow. Without focused validation, it would be easy to regress feature activation, runtime behavior, suppressions handling, or failure semantics while moving definitions from toolchain ownership into policy ownership.

Dedicated coverage is needed to prove that sanitizer behavior still works after migration and that the new policy-owned model is reusable and reliable.

Acceptance Criteria (DoD)

• Test coverage exists for the migrated sanitizer behavior in score_cpp_policies.
• Positive validation demonstrates that supported sanitizers can be enabled through the migrated integration model.
• Negative validation demonstrates that sanitizer failures are still surfaced correctly.
• Coverage includes sanitizer-specific runtime policy where relevant, such as suppressions or wrapper behavior.
• Tests clearly exercise the policy-owned feature path, not only legacy toolchain-local behavior.
• Validation is sufficient to catch regressions in feature activation and expected runtime outcomes.

How

• Review existing sanitizer tests and identify what currently validates behavior through score_bazel_cpp_toolchains.
• Add or update tests so they validate the migrated policy-owned feature definitions.
• Cover both successful activation and expected failure cases for supported sanitizers.
• Ensure tests exercise the integration points that downstream consumers will actually use.
• Confirm that suppressions, wrapper logic, and runtime environment behavior still work as intended after migration.
• Close any gaps where legacy tests only validated toolchain-local ownership rather than policy-owned behavior.