feat(actions): keep one arc-job-report comment per PR with a section per workflow

[JesperTerkelsen]

Summary

This lets the ARC job report cover several workflows in one comment per PR, so it can be turned on for monta-app/server.

monta-app/server runs its ARC jobs in four push-triggered workflows: Unit tests, Integration tests, Static checks and Smoke tests. The report kept one block per comment, so each workflow's run would overwrite the others' job table and mix their run histories.

• One section per workflow. Each section starts with <!-- workflow: <name> --> and its summary line now names the workflow (Unit tests · ✅ 6m 20s · …). A run re-renders only its own section, so each section keeps its own job table, suggestions and run history.
• The 50-run history is split evenly between sections. With four workflows that's 12 runs each, which keeps the comment well under GitHub's 65,536-character limit.
• Existing comments are adopted. monorepo-typescript and service-ocpi comments from before this change become the reporting workflow's section, with their history kept.
• A run whose commit has no PR now exits 0 with a message instead of failing. Push-triggered workflows on a branch without a PR are normal on server, and monorepo-typescript had a red report run from it (https://github.com/monta-app/monorepo-typescript/actions/runs/38420337388).

Testing

• python3 -m unittest discover -s .github/actions/arc-job-report -p 'test_*.py': 18 tests (including fix(actions): group "(shard N)" jobs into one arc-job-report row #383). The new ones cover two workflows sharing a comment with only the reporting section replaced, re-rendering the same run giving an identical comment, adopting an existing comment, and the history being split between sections.
• I built the combined comment for https://github.com/monta-app/server/pull/27724 from its Unit tests, Integration tests and Static checks runs for commit b293f11 (dry run, nothing posted). That gives three sections, each with its own wall time, jobs and history. After rebasing onto fix(actions): group "(shard N)" jobs into one arc-job-report row #383, Unit tests shows unit-tests · shards 1–8 in one row, and Integration tests lists both matrix jobs by full name, each with a suggestion (8cpu → 4cpu, fits).
• --dry-run on https://github.com/monta-app/monorepo-typescript/actions/runs/38420988227 adopts the existing comment https://github.com/monta-app/monorepo-typescript/pull/11599#issuecomment-6141491902 as a Pull Request - Main section with all 31 history rows.

Gaps

• monta-app/server's report workflow is a separate PR. It should merge after this one; otherwise its four workflows overwrite each other's comment.

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