Type validator request JSON recursively

[adamtheturtle]

Represent decoded validator payloads with a recursive JSON type instead of dict[str, Any]. Narrow string fields through strict adapters, make width validation narrow directly, and compare VuMark instance IDs with the empty string explicitly. This removes four inline type suppressions while making downstream assumptions executable.\n\nValidation: every applicable pre-commit and pre-push hook passes.

[adamtheturtle]

Consolidated into #3590 so this cleanup is represented by one PR for the project.