[adamtheturtle]

Shared Model Target request fixtures and response parsing imported the recursive JSON type from mock_vws.model_target, tying reusable API tests to the implementation they verify. Use Pydantic's public JsonValue type instead, retaining recursive JSON validation through the existing dependency.

Towards #3738. This removes all mock_vws imports from these two shared helpers; the rest of the contract-suite decoupling remains separate.

Validation:

• Existing Model Target API, failure-response and retry tests: 536 passed, 244 real-backend cases skipped.
• Nine offline JSON round trips covering primitives, arrays, objects and nested values passed.
• Pre-commit and pre-push hooks passed, including partition checks and all type checkers. Pylint passed for both changed modules.
• git diff --check passed.

Based on current main. The changed files are disjoint from open PRs #3769 and #3770 and the separate server-interface PR. Hosted CI is pending.