ci: add scheduled dispatcher for stable branch tests

[johnramsden]

Description

GitHub only fires schedule: triggers on the default branch, so squid (and any future stable branch) gets no periodic CI signal — the CI health dashboard's CE144 flaky-rate calculation counts only push/schedule/workflow_dispatch runs, and squid saw just 2 pushes in the last 60 days, so it usually shows "no data".

Adds .github/workflows/scheduled-branch-tests.yml: runs Mon/Thu 02:00 UTC (plus manual dispatch), fanning out gh workflow run tests.yml --ref <branch> per entry in a matrix (currently just squid; add future stable branches to the matrix as needed).

workflow_dispatch events triggered via the default GITHUB_TOKEN are exempt from the Actions recursion guard, so no PAT is required — only permissions: actions: write.

Depends on #791 — that PR adds the workflow_dispatch trigger to squid's tests.yml; dispatch from this workflow will fail with "workflow does not have a workflow_dispatch trigger" until it merges.

Type of change

• New feature (non-breaking change which adds functionality)

How has this been tested?

After both PRs merge: gh workflow run scheduled-branch-tests.yml --repo canonical/microceph --ref main should spawn a Tests run on squid. Confirmed via:

gh api 'repos/canonical/microceph/actions/runs?branch=squid&event=workflow_dispatch&per_page=5' --jq '.workflow_runs[] | {name, event, head_branch, created_at}'

No dashboard config changes needed — it already monitors both main and squid.

Contributor checklist

• self-reviewed the code in this PR
• added code comments, particularly in less straightforward areas
• checked and added or updated relevant documentation
• checked and added or updated relevant release notes
• added tests to verify effectiveness of this change

[sabaini]

lgtm