Fix immutable field validation error when a sub-schema is not Pydantic

[kenafoster]

Reference Issues or PRs

Fixes #2767

What does this implement/fix?

Put a x in the boxes that apply

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds a feature)
• [ ] Breaking change (fix or feature that would cause existing features not to work as expected)
• [ ] Documentation Update
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no API changes)
• [ ] Build related changes
• [ ] Other (please describe):

Testing

• [X] Did you test the pull request locally?
• [X] Did you add new tests?

How to test this PR?

Deploy Nebari with a configuration that includes an "overrides" or "values" block whose structure is not defined in the schema (this occurs when passing values to Helm charts). One example is:

jupyterhub:
  overrides:
    singleuser:
      extraEnv:
        ENV: "test"

Change the value of the "ENV" key and redeploy. (Nebari 2024.9.1 would error out)

Any other comments?