ci: split Tests workflow into 4 parallel shards via pytest-split

Target: <2min CI test wall time.

Runs the Tests workflow as a 4-way matrix instead of one job. Each
shard runs ~3,000 tests on its own ubuntu-latest runner (4 cores) with
-n auto xdist inside. Total effective parallelism: 16 workers across
4 machines (vs 4 workers on 1 machine today).

Was previously tried in #11566 and closed — shard 3 hung at 97% complete
for 100+ seconds with dozens of E/F markers. Root cause was cross-test
pollution exposed by splitting test files across shards (e.g. the three
test files that mutated sys.modules['dotenv'] at import time poisoned
whichever shard they landed in). That's now fixed by #11453 and #11577:
conftest is hermetic, the dotenv stub bombs are removed, and tests no
longer depend on each other's env-var side effects.

Changes:
- pyproject.toml: add pytest-split>=0.9,<1 to dev extras
- .github/workflows/tests.yml: 'test' job becomes matrix-split into 4
  groups with fail-fast: false. Runs 'pytest --splits 4 --group N'.
  pytest-split composes with -n auto from pyproject addopts.

e2e job is unchanged (already small, 20s).

Expected timing:
  Before: ~4m total (243s test step + ~25s setup)
  After:  ~90-115s total (shard wall time ~60-90s + ~25s setup)

Hash-based split is deterministic; no .test_durations file needed yet.
Can add one later via --store-durations for better shard balance.

.github/workflows/tests.yml

@@ -16,8 +16,13 @@ concurrency:
 
 jobs:
   test:
+    name: test (${{ matrix.group }}/4)
     runs-on: ubuntu-latest
     timeout-minutes: 10
+    strategy:
+      fail-fast: false
+      matrix:
+        group: [1, 2, 3, 4]
     steps:
       - name: Checkout code
         uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5  # v4
@@ -37,10 +42,11 @@ jobs:
           source .venv/bin/activate
           uv pip install -e ".[all,dev]"
 
-      - name: Run tests
+      - name: Run tests (shard ${{ matrix.group }}/4)
         run: |
           source .venv/bin/activate
-          python -m pytest tests/ -q --ignore=tests/integration --ignore=tests/e2e --tb=short -n auto
+          python -m pytest tests/ -q --ignore=tests/integration --ignore=tests/e2e --tb=short \
+            --splits 4 --group ${{ matrix.group }}
         env:
           # Ensure tests don't accidentally call real APIs
           OPENROUTER_API_KEY: ""