DEP: Deprecate casepath from argument use SUMO_CASEPATH instead

[tnatt]

Resolves #1812

PR to start prioritizing the <SUMO_CASEPATH> variable as the casepath for WF_CREATE_CASE_METADATA if present.
<SUMO_CASEPATH> is required by the SUMO_UPLOAD forward model, so it can reasonably be assumed to be defined for cases that upload assets to Sumo. The workflow now prioritizes this variable over the legacy casepath argument.

The logic is:

1. If <SUMO_CASEPATH> is defined:

• Use <SUMO_CASEPATH> as the case path.
• If the casepath argument was provided inform that the argument input is ignored and can safely be removed.

2. If <SUMO_CASEPATH> is not defined:

• If --sumo is set, raise an error instructing the user to define <SUMO_CASEPATH> in the ERT configuration.
• Otherwise, fall back to the provided casepath argument.

3. If neither <SUMO_CASEPATH> or casepath argument can be resolved:

• Raise an error explaining that <SUMO_CASEPATH> must be defined.

Note

I did not include an update to the main documentation now, since we are expecting some traffic of people following the getting started guide. I'll make a PR to update the documentation that we can merge once this change goes to komodo stable.

Checklist

• Tests added (if not, comment why)
• Test coverage equal or up from main (run pytest with --cov=src/ --cov-report term-missing)
• If not squash merging, every commit passes tests
• Appropriate commit prefix and precise commit message used
• All debug prints and unnecessary comments removed
• Docstrings are correct and updated
• Documentation is updated, if necessary
• Latest main rebased/merged into branch
• Added comments on this PR where appropriate to help reviewers
• Moved issue status on project board
• Checked the boxes in this checklist ✅

[tnatt]

Moved the validation of the casepath to this new _resolve_casepath so that we minimize the risk of copying .fmu to an incorrect location.