Move validation tests to plugin-validator package

- Add comprehensive validation tests to plugin-validator (uniqueness, business rules, cross-field, state machine, triggers, severity)
- Remove validation.test.ts from platform-node (tests now in correct package)
- Platform-node tests pass (5 tests) - focused on platform responsibilities
- Plugin-validator now has 71 tests total (some failing due to incomplete implementation - to be fixed separately)

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Copilot