[ci] Run downstream device-builder tests against PR Python code

Add a CI job that checks out esphome/device-builder@main, installs it,
overlays the PR's esphome via `pip install -e ./esphome`, and runs its
pytest suite. Gated by determine-jobs.py: only runs when Python files
under esphome/ change or runtime deps (requirements.txt, pyproject.toml)
change -- the surface device-builder consumes when it imports esphome.

This catches downstream breakage at PR time instead of after release,
mirroring the yarl -> aiohttp pattern.

.github/workflows/ci.yml

@@ -136,6 +136,44 @@ jobs:
           if-no-files-found: ignore
           retention-days: 14
 
+  device-builder:
+    name: Test downstream esphome/device-builder
+    runs-on: ubuntu-24.04
+    needs:
+      - common
+      - determine-jobs
+    if: needs.determine-jobs.outputs.device-builder == 'true'
+    steps:
+      - name: Check out esphome (this PR)
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          path: esphome
+      - name: Check out esphome/device-builder
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          repository: esphome/device-builder
+          ref: main
+          path: device-builder
+      - name: Set up Python
+        uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v6.2.0
+        with:
+          python-version: "3.13"
+          cache: pip
+          cache-dependency-path: |
+            device-builder/pyproject.toml
+            esphome/requirements.txt
+      - name: Install device-builder + esphome from PR
+        run: |
+          python -m pip install --upgrade pip
+          # Install device-builder with its esphome + test extras first so its
+          # pinned versions of pytest/etc. land, then overlay the PR's esphome
+          # so the downstream tests run against this PR's Python code.
+          pip install -e './device-builder[esphome]' -e './device-builder[test]'
+          pip install -e ./esphome
+      - name: Run device-builder pytest
+        working-directory: device-builder
+        run: pytest -q --maxfail=5 --durations=10 --no-cov --ignore=tests/benchmarks
+
   pytest:
     name: Run pytest
     strategy:
@@ -204,6 +242,7 @@ jobs:
       clang-tidy-mode: ${{ steps.determine.outputs.clang-tidy-mode }}
       python-linters: ${{ steps.determine.outputs.python-linters }}
       import-time: ${{ steps.determine.outputs.import-time }}
+      device-builder: ${{ steps.determine.outputs.device-builder }}
       changed-components: ${{ steps.determine.outputs.changed-components }}
       changed-components-with-tests: ${{ steps.determine.outputs.changed-components-with-tests }}
       directly-changed-components-with-tests: ${{ steps.determine.outputs.directly-changed-components-with-tests }}
@@ -247,6 +286,7 @@ jobs:
           echo "clang-tidy-mode=$(echo "$output" | jq -r '.clang_tidy_mode')" >> $GITHUB_OUTPUT
           echo "python-linters=$(echo "$output" | jq -r '.python_linters')" >> $GITHUB_OUTPUT
           echo "import-time=$(echo "$output" | jq -r '.import_time')" >> $GITHUB_OUTPUT
+          echo "device-builder=$(echo "$output" | jq -r '.device_builder')" >> $GITHUB_OUTPUT
           echo "changed-components=$(echo "$output" | jq -c '.changed_components')" >> $GITHUB_OUTPUT
           echo "changed-components-with-tests=$(echo "$output" | jq -c '.changed_components_with_tests')" >> $GITHUB_OUTPUT
           echo "directly-changed-components-with-tests=$(echo "$output" | jq -c '.directly_changed_components_with_tests')" >> $GITHUB_OUTPUT
@@ -1063,6 +1103,7 @@ jobs:
       - clang-tidy-nosplit
       - clang-tidy-split
       - determine-jobs
+      - device-builder
       - test-build-components-split
       - pre-commit-ci-lite
       - memory-impact-target-branch