ci: allow manual Tests runs on main via workflow_dispatch

[Jammy2211]

What

Adds workflow_dispatch: to the Tests workflow (.github/workflows/main.yml), so a run against main HEAD can be re-requested on demand. One file, +8 lines (6 of them an explanatory comment).

No change to what is tested, or to when CI runs automatically — push to main and pull_request triggers are untouched.

Why

The GitHub Actions outage on 2026-08-06 dropped the push triggers for several merges to main. The result is that main HEAD (7fe795c) carries no Tests conclusion at all, and neither does any commit back to f07dff1:

commit	on main	Tests run
7fe795c (merge #559)	tip	none
4c0680f	✓	none
66fe21c (merge #557)	✓	none
bfc766e (merge #558)	✓	none
e3a8c60	✓	none
8038d1c	✓	none
f07dff1 (merge #556)	✓	✅ success

That span is not docs-only — it contains a geometry_profiles.py behaviour fix (inverse reference-frame transform startswith → endswith), changes to cosmology/model.py, an astropy cap drop in pyproject.toml, and 31 lines of new tests.

Heart's ws_ci gate reads main-HEAD conclusions, so missing evidence blocks a release exactly as a red run would.

Recovery was awkward because neither existing lever works:

• re-running an existing run re-tests that run's own (older) sha, not the current tip;
• the only remaining trigger was pushing an empty commit to main.

workflow_dispatch gives a direct way to re-request a main-HEAD run after any dropped-trigger event.

Note

This PR's own pull_request run also exercises main's current content, which supplies the missing evidence for the untested span above.

---

Generated by Claude Code