Validate Model Target error JSON

[adamtheturtle]

Summary

• add a recursive JSON value type to the shared JSON validation helpers
• recursively validate decoded objects and arrays at the untrusted response boundary
• reuse those helpers for Model Target error envelopes, nested detail arrays, and string fields
• remove eight Pyrefly suppressions

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)

[adamtheturtle]

Consolidated into #3219 so this cleanup is represented by one PR for the project.