ci: split the Python tests over five shards instead of four

.github/workflows/pr_code_changes.yaml

@@ -162,7 +162,7 @@ jobs:
         run: uv pip install policyengine --system
       - name: Test documentation builds
         run: make documentation
-  # The YAML tests, the Python tests on four runners, and the check that
+  # The YAML tests, the Python tests on five runners, and the check that
   # every test ran once and passed, in the locked environment. As one job
   # they took 59 minutes on 10 October 2026 (run 38075384588), and runs were
   # cancelled at a 60-minute limit (issues #2099 and #2257). The release

.github/workflows/test-suite.yaml

@@ -4,10 +4,10 @@
 # publish. All three run the same jobs in the same locked environment.
 #
 # Jobs: the YAML policy tests (which also list every Python test), the Python
-# tests split over four runners, and Result, which fails unless every listed
+# tests split over five runners, and Result, which fails unless every listed
 # Python test ran in exactly one shard and passed, apart from the outcomes
 # accepted in .github/test_outcome_allowlist.toml. A caller that names its job
-# Test gets the checks "Test / YAML", "Test / Python K/4" and "Test / Result";
+# Test gets the checks "Test / YAML", "Test / Python K/5" and "Test / Result";
 # "Test / Result" is the one to require.
 
 name: Test suite
@@ -122,17 +122,23 @@ jobs:
     # the 16 GB runners peaked at 15.6 GB, 283 MB short of running out
     # (run 38156967136; two shards of an earlier run lost their runners).
     # With four, each worker runs about a quarter fewer tests.
+    #
+    # Five, not four, for time: on 20 and 21 October shard 1/4 took 50 to
+    # 52 minutes on unrelated pull requests, close to the timeout. A pull
+    # request adding about a fifth more parameters (#1855) hit the timeout
+    # on shard 1/4 twice, and its other shards took 48 to 59 minutes (run
+    # 39568525017).
     name: Python ${{ matrix.shard }}/${{ strategy.job-total }}
     if: ${{ !inputs.skip }}
     runs-on: ubuntu-24.04-arm
     # The shards took 15, 20 and 15 minutes on 10 October (run
-    # 38085920460). 60 minutes leaves room for a slow runner and still
-    # stops a hung shard.
+    # 38085920460), and up to 52 minutes with four shards on 21 October.
+    # 60 minutes leaves room for a slow runner and still stops a hung shard.
     timeout-minutes: 60
     strategy:
       fail-fast: false
       matrix:
-        shard: [1, 2, 3, 4]
+        shard: [1, 2, 3, 4, 5]
     steps:
       - name: Checkout repo
         uses: actions/checkout@v6

Makefile

@@ -21,7 +21,7 @@ test-yaml:
 
 # Every Python test, serially. To spread them over four cores add
 # PYTEST_ARGS="-n 4 --dist worksteal" (pytest-xdist); to run what one CI
-# shard runs, add "--shard K/4" (policyengine_uk/tests/conftest.py). CI runs
+# shard runs, add "--shard K/5" (policyengine_uk/tests/conftest.py). CI runs
 # these tests through .github/workflows/test-suite.yaml, in the locked
 # environment (uv sync --frozen --extra dev), and also checks each test's
 # outcome against .github/test_outcome_allowlist.toml.

docs/engineering/skills/testing.md

@@ -86,10 +86,10 @@ the same suite,
 tree; a release tests the wheel it is about to publish, installed on its own.
 
 The suite runs the YAML policy tests in one job and splits the Python tests
-over four runners. Each runner runs one shard:
+over five runners. Each runner runs one shard:
 
 ```bash
-pytest policyengine_uk/tests -n 4 --dist worksteal --shard K/4 -o xfail_strict=true
+pytest policyengine_uk/tests -n 4 --dist worksteal --shard K/5 -o xfail_strict=true
 ```
 
 `--shard K/N` is defined in `policyengine_uk/tests/conftest.py`. It keeps the