ci: run the Omnilingual-GAIA2 test suite

gaia2-cli/mt ships 94 offline tests that ran nowhere. Adds a gaia2-cli-mt-tests
job alongside the existing cli and runner jobs.

The job collects the whole `gaia2_mt` package rather than `gaia2_mt/tests`,
which would have picked up only 27 of the 94 — the 67 TermTable contract tests
live under `gaia2_mt/translation/contract/tests/`.

Author: caciolai

.github/workflows/on_push_py.yaml

@@ -179,3 +179,20 @@ jobs:
       - name: Run gaia2-cli runner tests
         working-directory: ${{ env.WORKING_DIR }}/gaia2-cli/runner
         run: uv run --locked --extra dev pytest -v tests
+
+  gaia2-cli-mt-tests:
+    runs-on: ubuntu-latest
+    needs: [lock-file]
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+      - name: Download artifact
+        if: inputs.use-artifact != ''
+        uses: actions/download-artifact@v4
+        with:
+          name: ${{ inputs.use-artifact }}
+          path: ${{ inputs.working-directory }}/oss-content
+      - uses: ./.github/actions/setup-python-uv
+      - name: Run Omnilingual-GAIA2 tests
+        working-directory: ${{ env.WORKING_DIR }}/gaia2-cli/mt
+        run: uv run --locked --extra dev pytest -v gaia2_mt