ci: run pytest in CI; complete dev requirements; unblock socket test

The suite was never run in CI (only hassfest/HACS), and requirements-dev
listed only pytest + smartthings-local while conftest.py needs the full
Home Assistant test harness.

- requirements-dev.txt: add pytest-homeassistant-custom-component (pulls
  in home-assistant + pytest + pytest-socket) and the integration's
  runtime deps (cbor2, pyOpenSSL, cryptography) needed to import it.
- .github/workflows/validate.yml: add a Pytest job on Python 3.14
  (current Home Assistant's floor) that installs requirements-dev and
  runs the suite.
- test_config_flow.py: the UDP liveness-sweep test needs real loopback
  sockets, which the HA harness blocks by default; take the pytest-socket
  socket_enabled fixture so it runs instead of erroring.

Full suite: 249 passed.

.github/workflows/validate.yml

@@ -35,3 +35,23 @@ jobs:
         uses: hacs/action@main
         with:
           category: integration
+
+  tests:
+    name: Pytest
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout the repository
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.14"
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements-dev.txt
+
+      - name: Run tests
+        run: pytest tests/ -q