broadcast: reject truncated cycle points

[oliver-sanders]

• Closes Broadcasting to a truncated cycle point can bring down a workflow #6407

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[wxtim]

Looks sane. Have tried it and it appears to fix the bug in question.

Presumably waiting for release stuff to finish before adding a changelog?

[oliver-sanders]

Presumably waiting for release stuff to finish before adding a changelog?

Waiting for a PR number (couldn't be bothered to guess).

[MetRonnie]

This also prevents traceback when doing e.g. cylc hold <future-task> using a truncated cycle