ci(experimental): cancel superseded runs on main (#1903)

[JarryShaw]

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• Followed the coding style (make pylint, make mypy, make isort)

• make test passes, and a test case covers the change. Narrow runs only, listed below.

• Added a changelog entry under docs/source/changelog/ and regenerated CHANGELOG.md, if the change is user-visible: N/A — centralised in docs(changelog): shared 1.5.0 changelog — long-lived, merges last (#610, #616, #617, #618, #620) #657

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This is option 1 from #1903: Experimental Tests now cancels superseded runs on a push to main, as it already does on a pull request.

• experimental.yml: pull requests and pushes to main share a cancelling group per ref. Any other event, should one ever be added, keeps a group of its own by run id and is never cancelled.
• unit-tests.yml: unchanged, so it still runs once per push, because its unittest-ordering leg runs only on main.
• Docs: docs/source/contributing/workflows.rst states the new rule.
• New test: tests/project/test_experimental_concurrency_1903.py evaluates the concurrency expressions with the repo's existing evaluator, for pull request, push to main, push to another branch, schedule, workflow_dispatch and workflow_run, and pins unit-tests.yml's block unchanged. It fails on main.
• Updated test: test_sweeps_on_latest_only_1732.py dropped its pin on the old expression text.
• What else reads these runs: no workflow_run consumer, required check, board sync or util/ci_timing.py path treats a cancelled Experimental run as red.

tests/project and the tier guard pass.

Closes #1903

[JarryShaw]

Verdict: GOOD TO GO at 1877248ff. A Sonnet cross-review confirmed the following:

• The concurrency expression: it gives pull requests experimental-tests-refs/pull/N/merge and pushes to main experimental-tests-refs/heads/main, both with cancel on. Every other event gets experimental-tests-<run_id> with cancel off. github.ref is never empty on the cancelling branches, so the && github.ref || github.run_id idiom cannot fall through to the run id.
• Unit Tests unchanged: unit-tests.yml is byte-identical to main.
• No other consumers: no workflow_run consumer, required check (Required checks passed and the Compat legs), util/ci_timing.py path or board sync treats a cancelled Experimental run as red.
• Tests: the new test evaluates the expressions with the repo's &&/|| evaluator and fails on main. The ci(python): the GraalPy experimental leg reaches 26% of the suite before its 25-minute cap #1732 test edit drops only the old-text pin. tests/project and the tier guard pass.

GitHub's concurrency docs confirm that cancel-in-progress: true also cancels a run already in progress in the group, and that it accepts an expression.