Cleanup assert() & AssertionError definitions

Editor assessment

The subject says the assert() and AssertionError definitions were cleaned up, naming the artifacts but not the problems: duplication, inconsistent messages, or misplaced definitions are all unstated. Type refactor is accurate; no scope token is present. No body describes the cleanup or its benefit, and no footers appear beyond the PR number. The named identifiers keep the message searchable and honest, but it teaches nothing about the state of the code before or after.