Edit Runtime: handle changes to task run mode schema

[MetRonnie]

[!IMPORTANT] Do not bump to a later milestone

Workaround for https://github.com/cylc/cylc-flow/pull/6554#discussion_r1926921905

Check List

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [x] Tests are included
• [x] Changelog entry not needed as this will just maintain continuity for users
• [x] No docs needed

[MetRonnie]

Now that https://github.com/cylc/cylc-flow/pull/6554 has been merged, this should be prioritised for the next release