test: add unit test infrastructure (Docker tester stage + CI)

- Add Dockerfile 'tester' stage (FROM builder):
  - Symlinks /usr/local/cuda/lib64/stubs/libcuda.so → libcuda.so.1
    so the test binary can satisfy the dynamic linker without a real GPU
  - Runs `cargo test --release` reusing the cached release build artifacts
    (no recompilation — tests complete in ~6s)
  - docker build --target tester . to run all 30 unit tests

- Add 'test' job to .gitea/workflows/docker-build.yml:
  - Runs before build-and-push (build-and-push needs: test)
  - Builds --target tester with registry build cache
  - Gate: build-and-push only runs when all tests pass

- Add run_tests.sh convenience script for local use:
  - Accepts optional test name filter as first argument
  - Respects CUDA_VERSION / UBUNTU_VERSION env overrides

All 30 unit tests pass:
  error::tests     — 7 tests (OOM detection, ModelNotReady HTTP shape)
  models::tests    — 17 tests (state machine, serialization, retry-after)
  worker::tests    — 6 tests (chunk ranges, silence snap/trim)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.gitea/workflows/docker-build.yml

@@ -18,7 +18,30 @@ env:
   UBUNTU_VERSION: ${{ vars.UBUNTU_VERSION || '22.04' }}
 
 jobs:
+  test:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Run unit tests
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: ./Dockerfile
+          target: tester
+          push: false
+          build-args: |
+            CUDA_VERSION=${{ env.CUDA_VERSION }}
+            UBUNTU_VERSION=${{ env.UBUNTU_VERSION }}
+          cache-from: type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:buildcache
+
   build-and-push:
+    needs: test
     runs-on: ubuntu-latest
 
     steps: