ci: fold test_transformers into the sharded nox job (#692)

## Summary
- Drop the standalone `transformers-py` job; `test_transformers` now
runs alongside every other session under the sharded `nox` matrix.
- Move the Hugging Face model cache into the `nox` job so shards still
avoid re-downloading models across runs.
- Update `checks-passed` to no longer gate on `transformers-py`.

## Test plan
- [ ] CI on this PR: confirm `test_transformers(...)` lands in some
shard and passes on Linux.
- [ ] Watch Windows shards — this is a scope expansion vs. the old
Ubuntu-only job. If they flake, add a `session.skip` for `sys.platform
== "win32"` in `noxfile.py:test_transformers`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- sfk:created-approved-by -->
Created by @AbhiPrasad

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[Slack
thread](https://braintrustdata.slack.com/archives/C0AQDETAVT3/p1786736441809709?thread_ts=1786736441.809709&cid=C0AQDETAVT3)

Co-authored-by: Starfolk <noreply@starfolk.ai>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: 3 people

.github/workflows/checks.yaml

@@ -131,40 +131,18 @@ jobs:
           dir="$HOME/.cache/braintrust/livekit-server"
           mkdir -p "$dir"
           echo "BRAINTRUST_LIVEKIT_SERVER_DIR=$dir" >> "$GITHUB_ENV"
-      - name: Run nox tests (shard ${{ matrix.shard }}/6)
-        shell: bash
-        run: |
-          mise exec python@${{ matrix.python-version }} -- python ./py/scripts/nox-matrix.py ${{ matrix.shard }} 6 \
-            --exclude-static-checks \
-            --exclude-session-prefix test_transformers
-
-  transformers-py:
-    runs-on: ubuntu-24.04
-    timeout-minutes: 30
-    strategy:
-      fail-fast: false
-      # Serialize jobs so later Python versions can restore the model cache
-      # saved by the first job instead of downloading the same models in parallel.
-      max-parallel: 1
-      matrix:
-        python-version: ["3.10", "3.11", "3.12", "3.13", "3.14"]
-    steps:
-      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
-      - name: Setup Python environment
-        uses: ./.github/actions/setup-python-env
-        with:
-          python-version: ${{ matrix.python-version }}
       - name: Cache Hugging Face models
         uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
         with:
           path: ~/.cache/huggingface/hub
           key: huggingface-transformers-${{ runner.os }}-${{ runner.arch }}-${{ matrix.python-version }}-${{ hashFiles('py/src/braintrust/integrations/transformers/test_transformers.py') }}
           restore-keys: |
             huggingface-transformers-${{ runner.os }}-${{ runner.arch }}-
-      - name: Test Transformers integration
+      - name: Run nox tests (shard ${{ matrix.shard }}/6)
         shell: bash
         run: |
-          mise exec -- uv run --project ./py nox -f ./py/noxfile.py -s test_transformers
+          mise exec python@${{ matrix.python-version }} -- python ./py/scripts/nox-matrix.py ${{ matrix.shard }} 6 \
+            --exclude-static-checks
 
   adk-py:
     uses: ./.github/workflows/adk-py-test.yaml
@@ -203,7 +181,6 @@ jobs:
       - static_checks
       - smoke
       - nox
-      - transformers-py
       - adk-py
       - langchain-py
       - upload-wheel
@@ -230,7 +207,6 @@ jobs:
           check_result "static_checks" "${{ needs.static_checks.result }}"
           check_result "smoke" "${{ needs.smoke.result }}"
           check_result "nox" "${{ needs.nox.result }}"
-          check_result "transformers-py" "${{ needs['transformers-py'].result }}"
           check_result "adk-py" "${{ needs['adk-py'].result }}"
           check_result "langchain-py" "${{ needs['langchain-py'].result }}"
           check_result "upload-wheel" "${{ needs['upload-wheel'].result }}"