ENH: Adapt to stricter project .fmu init behavior

[GibranAlfa]

Resolves #332

• POST /project/init now catches InvalidFMUProjectPathError and returns HTTP 422 when the target path exists but is not a valid FMU project root.
• Cleanup the test fixtures to properly setup project .fmu

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 ✅