Type and validate API JSON payloads

[adamtheturtle]

Summary

• represent Model Target request payloads with a recursive JSON value type
• recursively validate decoded Model Target error objects
• validate report response values before constructing public report models
• remove 16 Pyrefly suppressions without adding checker configuration

Validation

• uv run --group=dev prek run --all-files --hook-stage pre-commit
• uv run --group=dev prek run --all-files --hook-stage pre-push
• uv run --group=dev pytest -q (460 passed)