[MaxGhenis]

Fixes #2770

Reproducer: policyengine_uk/tests/code_health/test_ci_test_shards.py

The Python shards use almost all of the 75-minute job limit, so a slow runner can cancel a job whose tests pass. This adds a seventh shard, the fix test-suite.yaml documents.

Why

• Nine successful test runs started on 31 October (EDT) took 57 to 73 minutes a shard.
• In run 41191216150, first attempt, the limit cancelled shard 1/6 with its tests at 99%, and Result failed on the cancelled job.
• shard_durations.toml puts the listed slow tests at 134 minutes of worker time per shard on six shards and 115 on seven. The balanced split gives each of the seven shards 115 minutes.

What changed

• test-suite.yaml: the matrix gains a 7, and a new paragraph above the job records the numbers above.
• pr_code_changes.yaml, Makefile, docs/engineering/skills/testing.md and docs/book/validation/ubuntu-vs-macos.md: "six" and K/6 become "seven" and K/7.
• A changelog fragment.

The job name and --shard read strategy.job-total, and the required checks name only Test / Result and Lint, so no ruleset changes.

Checks

• test_ci_test_shards.py and test_ci_runner_variables.py pass: 126 tests.
• Run as --shard K/7 for K = 1 to 7, the same files select 19, 23, 16, 15, 13, 22 and 18 tests, which sum to 126, and all pass.

axiom: n/a: CI configuration only

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