[Enhancement]: Make case group an editable case level field that propagates to executions

### Is your feature request related to a problem?

Users may accidentally keep the `CASE_GROUP` configuration set in an e3sm run script to the incorrect case group value. This causes incorrect grouping on SimBoard.

### Describe the solution you'd like

`CASE_GROUP` is an execution level configuration derived from `env_run.xml`.

We need to have `CASE_GROUP` be editable at the case level and propagate to all the executions. Executions should all have the same case group. If a case has executions with different `CASE_GROUP`, then show the list of case groups and warn the user that overriding will propagate to all the executions.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

No files or tests are named. Start by tracing how CASE_GROUP is read from env_run.xml and represented at the case and execution levels. Confirm the case-level edit, propagation to every execution, and warning behavior when existing executions have different case groups.

Written by the indexing model from the issue text.