Add CI validation for latest_fusion/ schemas

## Problem

The `schemas/latest_fusion/` directory contains 7 auto-synced schema files that are merged via the `auto-approve-fusion-schema-sync-prs.yml` workflow without full CI validation coverage.

While the `validate-json-schemas` CI job compiles all `.json` files under `schemas/` (including `latest_fusion/`), the `test-against-sample-dbt-files` job only tests versions `1.5`, `1.6`, `1.7`, and `latest` — it does not include `latest_fusion` in the matrix. This means:

1. **No sample-file validation** — there are no test fixtures under `tests/latest_fusion/` to validate schemas against known-good and known-bad dbt files.
2. **Auto-merge without test coverage** — the auto-approve workflow approves and merges `fa-assistant` PRs without requiring the `test-against-sample-dbt-files` job to cover `latest_fusion`.

As a result, a structurally valid but semantically incorrect schema could be merged without detection.

## Suggested Fix

1. Add `latest_fusion` to the `dbt-version` matrix in the `test-against-sample-dbt-files` CI job.
2. Create `tests/latest_fusion/valid/` and `tests/latest_fusion/invalid/` directories with appropriate sample dbt files.
3. Consider adding a required status check so auto-merge only proceeds after CI passes.